Following impressions are based on listening with the GO450 as DAC, Leckerton UHA-6S.MkII as amp (high-gain implemented), and Spotify in offline mode as source since I’m currently without internet at my place. Because the UHA-6S.MkII served as my amp, I used the single-ended stock cable that came with the Omni. While the stock balanced cable would’ve worked with my EF-6, I’ve already packed that up so I can list it in the classifieds. The UHA-6S.MkII is my current reference amp for a reason. I prefer it to the EF-6 on several counts. My current reference headphone is the LCD-2.2. I also have an HE-500, HD800, Mad Dog and stock T50RP on hand. My taste in music leans toward the electronic.
 
Some non-sound related impressions. Weight seems right. I’m used to planars, however, so YMMV. Not too heavy but substantial-feeling, like you’re holding something that’ll last. Heirloom quality if taken care of. And you might want to heirloom them: These things are beautiful. Omnis arrived in lambskin; I switched to leather. Pads are plush but not overly so. I didn’t notice any sonic differences between the sets of pads. My primary interest was comfort. I’ll let others speak about “pad rolling”.
 
Overall, the comfort on these is outstanding. Easily on a par with the Alpha Dog, Paradox Slant and ZMF x Vibro. There’s a slight issue with getting the left cup to seal on my head. My head could be to blame, though, as I encounter this issue with other headphones. That said, it might be slightly more persistent with the Omni + cowhide pads. The lambskin pads seal perfectly. Strangely, of all the headphones I’ve tried, I think the LCD-2.2 might fit my head the best, with the possible exception of the HD800.
 
Omni is less isolating than the other T50RP mods I’ve heard. This isn’t surprising given its semi-open construction. Depending on the track playing, I could hold short, to-the-point conversations with a friend while we were listening together; he was wearing the HD800. I’m sure we were moderately yelling at each other. Also, the Omni soaks up a lot of power. I can max out the volume on my iPhone 6 without pain. It’s near or at the limit of comfortable but I can do it. This isn’t the case with, say, the LCD-2.2.
 
A few remarks on tonality.  While I wouldn’t say the Omni is a basshead headphone, the bass is excellent in quality and extension. When compared with the LCD-2.2, I have the sense of a slight recession in the mids. Voices seem a vaguely farther away. I’m not saying that the Omni has recessed mids; I’m saying that voices seem closer with the LCD-2.2, and perhaps fuller, more textured. An upshot is that the Omni is, from I’ve heard, completely free of glare in the high midrange, viz. in 5k-6k. The T50RP driver is supposed to have a tendency for glare. I could detect it on occasion with the Alpha Dog, though it was hardly enough count against that headphone. If you’re sensitive to glare in the high midrange, you needn’t worry about the Omni. The treble is relaxed without being rolled-off. There’s plenty of definition at 10k.
 
Some thoughts on technicalities. The HD800 is better at representing vast and distant sounds than is the Omni. That’s an obscure claim. It has to do with what might be termed the delicacy of a headphone. Vast, distant sounds are fragile and rich in information; it’s easy for this information – perhaps in the form of the felt texture of the sound – to get smoothed out, dulled. Of course, excellent treble resolution – e.g., in the “air” frequencies – contributes to this ability to represent vast distances, and the HD800’s treble resolution is unmatched in my experience. Let me give a concrete example. The synths – if that is what they are – that come in at 0:35 on Clark’s “There’s a Distance in You” from his eponymous album. (The significance of the title in the present context isn’t lost on me. This is an excellent track for testing a phone’s ability to render the timbre of vast distances. It’s also the shortest seven minute track I’ve heard.) With the HD800, you can hear how far away those synths really are. They’re not just quiet; they’re way off in the distance. This technicality might come at the cost of tonality: The HD800, while world class in treble resolution, is an almost unbearably bright headphone. I should point out that the low-end aspects of “There’s a Distance in You” sound much better on the Omnis.
 
Soundstaging and imaging are first-rate for a (semi)closed headphone. Among the headphones I have on hand, I would put the Omni between the LCD-2.2 and the Mad Dog with respect to width. The overall effect isn’t exactly holographic, but I suspect it’s difficult to get holographic imaging with (semi)closed headphones.
 
Greater bass impact/slam than my LCD-2.2 and HE-500, and greater than my friend’s Mad Dog. Given that I haven’t had the Alpha Dog and the Paradox Slant for several months now, I’ll forego a detailed comparison here. Suffice it to say, I don’t think the Omni is outshone by either. Bass impact/slam is only one aspect of an overall excellent transient response. This is a fast headphone. The transients aren’t as quick as with the HD800, but then, the Omni is a thicker-, fuller-sounding headphone.
 
Thanks again to Zach for putting this tour together! I’m honored to have been given the chance to listen to ZMF’s new flagship. It’s a solid piece of craftsmanship with a smooth, defined signature. 

Thanks to @Todd for putting together the loaner tour and for including me. @Todd’s contributions to the community are considerable. I bought one of my first higher-end headphones from him – a Grado PS500 – when I got into the hobby.
 
For a while now I’ve been looking for a headphone that strikes a balance between the LCD-2.2’s bass resolution and the HD800’s treble resolution. Given that the natural, realistic bass timbre of the LCD-2.2 can be attributed – I take it – to the planar driver, it’s possible that a dynamic headphone may not be able to recreate that timbre. That’s reason enough to think there might not be a perfect headphone for me. Although the HD800S does seem to improve on the HD800 as far as bass is concerned, while taming in some measure the lower-treble glare, it’s not my perfect headphone. That said, it’s nonetheless a truly remarkable headphone.
 
Primarily, I used my UHA-6S.MKII and a friend’s Benchmark DAC1 to drive the HD800S. Occasionally I switched to my EF-6 to experiment with the balanced cable. I want to put the cable question to one side. Still, I preferred the HD800S out of the Leck. If I need to adduce a reason for this preference, I’ll say that the EF-6 seemed to impart a bit too much upper-mid/lower-treble energy, or that the transients became a bit too etchy. It’s something like that – whatever it is that explains my preference. I should also note that I used Spotify as my source and a GO450 as my DAC, and that I tend to listen in the 70dB to 85dB range.
 
Bass
My understanding is that Sennheiser has worked to increase the amount of perceived bass in the HD800S by boosting harmonic distortion. From what I can tell, they were successful. A friend of mine – who happens to own the HD800 – and I directly A/B-ed the HD800S and the HD800. In this case, we used his Benchmark DAC1 as amp and DAC. We both thought there was an increased perception of bass presence. The increase was marginal, however, and did not create the impression of listening to a pointedly different signature. To me – and perhaps to my friend – the impression was that of listening to refined signature. Because I generally prefer a forward bass presence, I lean toward the HD800S here (although I wouldn’t describe the HD800S as “bass forward”).
 
6k spike
The HD800S’s putative drop in lower-treble energy wasn’t immediately obvious in direct A/B testing against the HD800. Neither I nor my friend had much to comment on this point. The decrease in energy was perceptible across longer, non-A/B listening sessions. This is shown, I think, in the fact that I’m actually able to listen without fatigue to the HD800S for longer periods of time than the HD800. This conclusion is, of course, subjective as it’s grounded in my personal tolerances. But I tend to be a bit sensitive to a hot treble; the fact that I’m not particularly sensitive to the HD800S makes me think the treble has been successfully moderated. And because I generally prefer a relaxed treble presence, I lean toward the HD800S here (although, again, I wouldn’t describe the HD800S as having a “relaxed treble”).
 
HD800S vs HD800
I think the supposed improvements of the HD800S over the HD800 make for a marginally better headphone. I say “marginally” so as not to overstate the difference between these two headphones. The HD800S is still obviously the HD800. If you like the HD800, I would find it surprising that you don’t like the HD800S. On the other hand, if you don’t like the HD800, I wouldn’t find it surprising that you like HD800S – provided that your problems with the HD800’s signature aren’t profound.  
 
HD800S vs. LCD-2.2
While there are dynamic headphones that can recreate the tonality of planar bass, I have yet to find a dynamic headphone that can successfully recreates its timbre. At the same time, I’m not sure I’ve heard a planar headphone that successfully recreates the HD800S’s (or the HD800’s) treble timbre. It’s entirely possible that choosing between the two will engage one’s personal preferences. In my case, I prefer planar bass timbre to dynamic treble timbre. It seems in my experience that one might not be able to enjoy both in a single headphone. If this is the case, I will naturally tend to prefer planar to dynamic headphones. If this isn’t the case, one day I will perhaps find a headphone that is, ceteris paribus, perfectly suited to my taste.
 
As it happens, the HD800S isn’t perfectly suited to my taste. By that I mean it isn’t the perfect headphone for all of the music I like. (Of course, what headphone is?) It doesn’t excel with EDM-leaning electronic music, although it does excel with IDM-leaning electronic music. It’s possible that I could come to prefer the HD800S to the LCD-2.2 with respect to IDM. In that event, I would have a use for both headphones. It’s probable, however, that I would come to see the HD800S as simply an alternative to the LCD-2.2 with respect to IDM. The HD800S would allow me a better view, as it were, of the upper register of music I already enjoy with the LCD-2.2, albeit with a worse view of the lower register.
 
I would be remiss not to mention something about the respective comfort of these headphones. While I don’t find the LCD-2.2 to be an uncomfortable headphone, the HD800S is far and away more comfortable. My ears touch no part of the headphone; it weighs noticeably less; it better retains its position through calm head-bobbing.
 
Conclusion
I can say that I prefer the signature of the HD800S to the HD800. This is on account of the marginal increase in perceived bass and the marginal decrease in energy in the 6k region. If I were to pick one, I would certainly pick the HD800S. The HD800 has been, and remains, a headphone that is to me just barely bright and barely thin. I’ve been able to enjoy it in small doses, short listenings. The HD800S allowed me to listen for 2+ hour sessions without fatigue.
 
Considering the unparalleled imaging and the resolution of detail of the HD800S (and, for that matter, the HD800), the headphone strikes me as a very solid choice. Of course one should first listen to other potential endgame headphones. If one is an orthohead (like myself), it’s possible that the HD800S’s sound – like practically all other dynamic headphones – will not be finally satisfying. I couldn’t own the HD800S by itself; the LCD-2.2 would be a necessary accompaniment. At the same time, I could own the LCD-2.2 by itself.
 
Supplemental: Sample music
boerd, Dwaal
Reso, Ricochet
Scuba, Update
Sieren, Transients of Light
Submerse, Awake
Tsaik, Lemon

Thanks to @TTVJ for the loaner. I was interested in the Dharma as a potentially bassier alternative to the HD800. While I do prefer the Dharma’s to the HD800’s bass, the Dharma’s treble and mids are dealbreakers. I prefer the HD800’s and the LCD-2.2’s treble presentation to the Dharma’s. I prefer the LCD-2.2’s bass and mids to the Dharma’s.
 
Spotify serves as my source. The GO450 serves as my primary DAC, although I occasionally used the iPhone 6’s internal DAC when listening to the Dharma. For amplification I use the UHA-6S.MKII or the EF-6. With the Dharma I also tried the iPhone 6’s internal amplifier; it performed surprisingly well given the Dharma isn’t demanding. My reference headphone is the LCD-2.2. The HD800 is a close second. In addition, I compared the Dharma to the Alpha Dog, the HE-500, and a T50RP (my girlfriend’s personal modded version). In the interest of concision, I’ll restrict my focus in the following to the LCD-2.2 and HD-800.
 
*          *          *
 
1. Bass
The Dharma’s bass has received considerable attention. The measurements are – as I’m sure everyone knows by now – less than ideal. Elevated distortion across the bass frequencies and extending into the mids. I didn’t hear any of this. A prominent commentator has said that the bass has a “furry fuzzy” texture to it. I’m not sure I know how to listen for that. Kiasmos’s track “Wrecked” on their 2012 Thrown nicely displayed the Dharma’s bass. Slam/impact was sufficient, although below the LCD-2.2’s levels. If I had a criticism of the Dharma’s bass, it would be that nothing about it really stands out. And that’s fine. A headphone doesn’t have to distinguish itself in every area of the frequency response.
 
A friend of mine who swears by the HD800 said of the Dharma: “This headphone has the bass I wish the HD800 had”. I do think the HD800 would be improved if its bass were at the Dharma’s levels. Maybe someone could claim that the HD800’s bass is objectively better. But if that means it’s better simply according to the measurements, I don’t see why that should matter very much. Perhaps one could argue that the HD800’s bass is more accurate, more neutral than the Dharma’s. Nonetheless, the Dharma’s bass doesn’t strike me as bloated, as inaccurate or colored.
 
2. Mids
It’s unfortunate that the first track I listened to on the Dharma was Saturn Never Sleeps’ “Bit by Bit” from their Yesterday’s Machine. This is a track I’m familiar with. I wouldn’t say it’s a test track but I’ve spent plenty of time with it. The Dharma is the only headphone on which it’s been sibilant. I don’t think the Dharma was revealing something about the track I’d never been able to hear before. Rather, I think the sibilance was an artifact of the headphone. I noticed this on a number of other tracks. From the beginning, then, I worried the Dharma might be malignantly sibilant. Subsequent experience tended to reinforce this worry. I think I would reject the Dharma as a headphone for this reason alone. I’m willing to forgive a headphone for coloration if the result is enjoyable, if it deepens one’s engagement with the music without compromising too much accuracy. But sibilance is painful, unpleasant coloration.
 
3. Treble
This is where I had my main problem with the Dharma. I’ll be very frank: The treble spikes hurt my ears. The discomfort starts almost immediately with the right (or, I should say, the wrong) music and is seriously fatiguing as I proceed. Now, I tend to be sensitive to boosts in the treble. The LCD-2.2 is my reference headphone for a reason; I naturally prefer darker-sounding headphones. I also prefer warmer-sounding headphones. The Dharma is certainly neither dark nor warm. I’m not sure I’d say the Dharma is a bright headphone. I am sure, however, that I would say it’s a spiky headphone. This is a phenomenon related to the sibilance mids. The frequency response across the treble is too drastically uneven for me to enjoy the Dharma without worrying about when the next peak will hit.
 
Transitioning from the Dharma to the HD800 brings home just how spiky that treble really is. I usually find the HD800 too bright. But the HD800 doesn't strike me as too bright after listening to the Dharma for around 10 minutes. I’m not prepared to say that the Dharma can’t sometimes get the treble right. There is sometimes an airy, spacious quality to the music. But you can get the same effect from with the HD800. So I don’t see what recommends the Dharma on this point.
 
4. Technicalities
Timbre can be a bit hollow, tin-canny. Occasionally ethereal-sounding. Vocals can be ghostly, haunting; that’s a good thing. Taja Sevelle’s Toys of Vanity, especially its title track and “Making Love to the Air” are exemplary on this point. Her voice seems to hang in midair. This is unfortunate since you’re almost immediately snapped out of it by sibilance, or simply a spike in the upper mids. If the upper mids to high frequencies were smooth, this could be a hell of a headphone.
 
The Dharma’s soundstaging was wider, more diffuse than the LCD-2.2’s. I still think the LCD-2.2 images at least as well, probably better. Given that the LCD-2.2 sounds more natural to me, more cohesive, than the Dharma, it has a more holographic or 3-D presentation. I’m able to hear where individual sounds are located in the total image, if it makes sense to put it that way. The Dharma, on the other hand, can sound a bit left and right. As with the HD800, soundstaging and separation with the Dharma can come across as artificial, as though details are really just artifacts of the headphone and not integrated parts of the image itself. I should note that – to my ears – the Dharma’s soundstaging and separation fall short of the HD800’s. The HD800 is still the king of soundstage. 
 
As far as resolution is concerned, I would rank the Dharma below the HD800 and the LCD-2.2 in that order. I’ve long felt that my LCD-2.2 resolves as well as any headphone I’ve heard, with the exception of the HD800. This isn’t to say the Dharma isn’t resolving. It just didn’t strike me as exceptionally resolving, especially when compared with other headphones similarly priced.
 
*          *          *
 
It’s anathema, I realize, to say something like this but I enjoyed the Dharma most when driven by the UHA-6S.MKII with the iPhone as source. The two devices were connected via a short analog cable, so it was technically double-amping. Double-amping doesn’t worry me terribly, at least given the amps involved, although I avoid it if I can. The native EQ in Spotify’s mobile app – specifically, the treble reducer – makes the Dharma less offensive. I should note – in the interest of full disclosure – that I find the Dharma almost unlistenable in any other configuration, at least for any significant length of time. The headphone is simply too bright, too spiky, for my ears. Anyone who is sensitive on these points should take caution when considering the Dharma as his next headphone.  
 
NB: The graphs provided in the "Review Details" section don't reflect this view. I'm not sure why the graphs are displayed that way given the values I set when publishing the review. 

I Background, gear used and manifesto
I first got interested in the ECP Black Diamond around the time of its release. I was in the market for a “flagship” amp but still didn’t know what type of headphones I preferred: Dynamic, Planar or Electrostatic. While I’ve yet to hear an electrostat, I’ve found so far that I prefer the planars to dynamics. When I subsequently opted to go full-planar (for home listening, at least) I somehow lost sight of the Black Diamond. Reading recently that it was a good match with the LCD-3, and finding a great deal for a third-hand model, I decided to take the plunge.
 
Before moving on I should point out that I don’t own the LCD-3. The headphones I’ve used when evaluating the Black Diamond have been the LCD-2.2 and the Alpha Dog. I’ve used the HRT microStreamer as a DAC. I don’t claim to be able to discern great differences among DACs because I haven’t yet developed the skills required for telling them apart. The microStreamer also served as a DAC for the Gustard H10, which will function in this review as the comparison amp to the Black Diamond. All relevant listening with the H10 was done with the +6dB pregain enabled. My music preferences, which can be found in my profile, will have to suffice for giving some impression of what I listened to since I don’t as yet have a set playlist of test tracks.
 
I should also point out that I don’t really know what I’m doing. This is the first “real” review I’ve attempted and I don’t yet have a feel for the form. I’m lucky enough to have money to spend on the audio gear I want, so I don’t really have the extra for measuring devices. I think providing measurements is an important contribution to our hobby but I leave it to others.
 
My focus is subjective phenomenal experience. Now, my using the word “subjective” doesn’t entail that I take my impressions to be subjective. Rather, I take myself to be giving a description of what anyone would hear if similarly positioned. I accomplish this by giving a description of what I hear. And no, that doesn’t mean I think I hear everything. What I mean is that I’m a human being equipped with the auditory capacities that are natural to our species. 
 
In other words, my review is subjective because it is grounded in first-personal experience. This experience, however, needn’t be understood as available necessarily to only a single person. You should, if you take up and listen, hear what I hear to the extent that I hear it. For that reason, what I have to say should, I hope, be of some use to everyone. All of that is contingent on my not being wrong, which of course, I could be.
 
II TL;DR, or my vague impressions of the Black Diamond
I have an overall impression of a slightly forward sound presentation, probably as a result of its intimate character. It’s this character that keeps the amp from coming across as aggressive. I’m not aware of any measurements for this amp but I’d say it strikes me as relatively neutral across the audible range. While it won’t win the coveted Blue Whale Award for Plankton Retrieval, the amp still furnishes enough detail to make for intelligent listening. The dynamism and the subtle timing cues contribute to the integrity of individual sounds and, if the music provides it, thereby make complex imaging possible.
 
III Ecce ampo
I don’t really know how to talk about a component by itself. Indeed, you don’t listen to, say, your amp and only your amp but your amp as it drives your headphones (and, I suppose, as it amplifies your source). Therefore, for the rest of the review I’m going to talk about the Black Diamond as it compares with Gustard’s H10 and as it pairs with specific headphones.
 
Comparison with the H10
I have said that in the thread dedicated to the Gustard H10 that it can go toe to toe with the Black Diamond. I mean that not as an insult to the Black Diamond but as a compliment to the H10. The ironfisted control the H10 has over planar drivers is astounding.
 
Similarities aside, the H10 doesn’t offer as pure an experience as does the Black Diamond. It’s as though Doug Savitsky meant the Black Diamond to be a resting place, or a stopping point, in your relation to the gear. Turn it on, plug in your headphones, set your volume and you’re done. The rest is music. No tube-rolling, no adjusting pregain, no EQing or tweaking the stereo image. You can finally stop fidgeting with your gear and simply listen.
 
Of course one might respond: “That’s all well and good but sound quality is the most important factor when evaluating an amp; so if the H10 matches the Black Diamond’s sound quality, then call it as it is.” But that would ignore large swathes of our interactions with audio gear, however, and would shows a limit in one’s aesthetic sensibility. It isn’t shallow to say, for example, that it matters how an amp looks. There is an elegance to how the design of the Black Diamond allows one to access the music. The design itself is a work of art.
 
I think the H10 has greater bass presence and texture. That isn’t to say that the Black Diamond’s bass is recessed; rather, it is defined and competent. Only there’s a certain lack in the presentation I find it difficult to put into words. A slight emphasis, when compared to the Black Diamond, on the low end and a vaguely warm signature. It’s perhaps for this reason that the signature of the Black Diamond is clearer, I find, than that of the H10. Perhaps I mean that the Black Diamond’s signature is more neutral than the H10’s. It’s easier to perceive all parts of a track on the Black Diamond. It isn’t as though I find the Black Diamond to have a clearer, more realistic timbre. Both amps yield excellent timbre with all of my headphones, although it’s worth noting I think all of my headphones have excellent timbre.
 
Headphone pairings
It might be surprising to read this (and it was certainly surprising to type it), but the Alpha Dog can actually sound dark on the Black Diamond, or at least darker than it ordinarily does. By no means a bright headphone – if bright is taken pejoratively – the Alpha Dog probably wouldn’t please listeners who prefer a veiled sound. Still, I think that it can take on a warm, syrupy texture when paired with the Black Diamond, e.g. on anything by Karma Kid, without compromising the “air”-frequencies.
 
The Alpha Dog might have the vastest soundstage of any closed headphone I’ve heard. The headphone is excellent at conveying a perceived sense of distance. This makes for an interesting experience with the intimate Black Diamond. I think there could be an apparent reduction in the expanse of the Alpha Dog’s soundstage in this pairing. But that is to be expected, and is welcome in order achieve the peculiar distancing effect.
 
The Black Diamond paired with the Audeze LCD-2.2 is one of the more musical combinations I’ve heard, maintaining respectable and engaging levels of plankton without being fatiguing and without compromising the overall coherence of the music. I think Maxvla described the Black Diamond as providing one of the more intimate experiences he has had with the HD800. I would say that the LCD-2.2 also sounds intimate with the Black Diamond, at least in the following sense. “Intimacy” can name a way of relating to an object that is marked by an effortless focus on and attention to that object. Keeping your eye, as it were, on the music as a whole is easy with the Black Diamond.
 
As it happens, Maxvla also describes the Black Diamond as “lacking in ultimate air”. But I think that contributes to the Black Diamond’s being an intimate amplifier. Ultimate air is part of what you sacrifice for intimacy. One doesn’t have a Godlike, all-comprehending perspective in intimacy. Listen to Braids’ Flourish // Perish or Long Arm’s The Branches (Deluxe Edition) on the Black Diamond, but not magisterial classical works.
 
IV Closing thoughts
As I write this I don’t know whether finally I’ll keep the Black Diamond, and if so, for how long. That isn’t the way to begin summing up what has been a very positive account of the Black Diamond. I can’t in good faith, however, claim that my search for a flagship amp – whether solid state or tube – has ended when my experience is still so narrow. I certainly hope I didn’t hear the Black Diamond too early.  
 
It's worth pointing out that the Black Diamond brand new costs x4 more than the H10 ($1600 to $400). This makes the H10 a considerably greater value, I think, than the Black Diamond from the perspective of price-to-performance. This doesn't take away from the sound quality of the Black Diamond, but it does give some indication of just how good an amp the H10 is, for planars at least. 
 
Given that the H10 provides nearly the same performance as the Black Diamond at the quarter of the price, odds are I'll keep the H10 longer than the Black Diamond. I wouldn't say owning both makes either superfluous. I can say this, however: I don't think I'd like to have the Black Diamond as my only amplifier. The purity of its experience is valuable in a context in which other components are able to pick up the slack.