Edit: see end for new perspective regarding two things: how it works as a preamp, and how it may be improved with a better power chord
 
Intro: I'm a music-lover, not an audiophile. Relatively new to this hobby. And I haven't extensively tested amps. I do extensive subjective listening tests with what I own, and that's about it. This review is not geared towards professional audio engineers who need to use an amp for long hours and are not trying to 'enjoy' the amp. For such people, this amp might actually be a good choice. I tend to listen many hours a day, too, so that's why I probably won't give up this amp. But I don't find it to be very enjoyable or engaging compared to other power sources with my Beyers.
 
1. It is known that when the BCL came out in 2004 they had been using Sennheiser's then top-of-the line phones to fine tune it.
 
A) Senheisser's HD6x0 series is great, as any visitor to Head-fi can attest through the millions of posts involving them (and the older 580 model). But they have several problems: lack of micro-detail, slow, and flabby bass (650), with highs that can bore some people.
 
B) The BCL is the opposite: tons of detail, fast, tight bass, and really nice extended highs. Most people are amazed when they try the BCL with the older flagship Senns. More recently, some have expressed amazement at first when pairing this with the new-ish 800 and 700 series, however, these are also known to have detail-oriented mids, which over time, can grow old with lack of feeling and character. That's why, from what I've read, a lot of people end up getting a warmer amp than the BCL, or even a high-end tube amp to go with the newer Sennheiser series--the BCL just isn't 'musical' enough for detail-monster headphones.
 
C) Soundstage is precise, but not huge, which is fine for most kinds of music. Noise level is low, which is something to appreciate especially for long listening periods.
 
2. Unfortunately the BCL is not amazing with Beyers, in my opinion.
 
A) because Beyer's already have fast bass, cool mids, and extended, controlled highs. I haven't listened with the T1, but with the DT990 and the DT150 (which is amazing!!) it doesn't really complement the strengths or compensate for the weaknesses of these phones.
 
B) I think if you have phones with really warm mids but need a big more juice up top and down below, this might be worth auditioning. I was blown away at first with how fast and detailed this amp was, but over a few months (about 5, with at least 100 hours of listening each month, mainly on through a HRT HD + external linear power supply source) I have gradually grown out of love with this amp.   
 
The sound is leaning towards the bright side, even though it's not harsh. I would not say it favors upper-mids, as female voices don't sound particularly good in my opinion. It's just rather bright, without being bass-light per se. 
 
C) The bass is interesting, but to me it doesn't have acoustic realism at all. It is tight to a fault. Hip-hop sounds funny, for example. The notes in the low end are clear and you know what they recording studio was trying to do, but it lacks a true visceral impact compared to more powerful amps (even the $99 Schiit Magni, for example). The visceral impact of bass is what makes bass, bass, to me. Even with classical and acoustic jazz (the majority of my music), low end instruments just don't sound as they should. You know what they're playing, but it doesn't sound live, it sounds recorded. Here, with a dark and bassy headphone like the HD650, you're probably going to feel different. You may find that the bass is perfect--and that's why I pointed out that for phones with good bass and sub-bass to begin with (but not ORTHOS!) this amp can provide extra precision and speed. But with phones that have strong, flat bass, or bass that's already pretty tight (like the Senn 700-800 or all the DT Beyers) I think you can do better. And although I've read that this amp is good for Mad Dogs and other efficient Ortho designs, it is still pretty low-powered (I believe 400mw per channel/60 ohms) compared to most newer solid state and tube amps that cater to the present day Ortho wave (i.e., 1.0 Watts per channel/50 Ohms for Schiit Magni 2).
 
Conclusion 1 (before edit): This mixed review could be construed as positive. However, the price of this amp, despite what it can do to noise floor, soundstage, and detail, does not seem to be justified. Especially since most phones really can be driven by lower costing amps, or stereo equipment that you need anyways for your speaker set up. Alas,, Lehmann should get credit for being one of the first really high quality headphone amps that does so much right for the Sennheiser 6x0 series. If you don't have those cans, but if you just really love hearing bright, clean detail all day long, don't care about engaging mids (or have phones with very warm, forward mids), and have an extra $1000 extra to spend on a headphone amp, this could be a good amp for you.
 
Conclusion 2: There are two things I hadn't fully explored, and I am open to the possibility that I will even raise the stars further from 3.5 to now 4.5, once I have a better source and/or better headphones.  A) the truth is it can do interesting things in your larger speaker sound system as a pre-amp. It provides a very fast, clear, and separated sound that can give life to a vintage system like the one I have. It's really amazing how it makes the sound a lot more punchy and extended in the highs and lows. I can see how, though, in very high end speakers, this might be too much of a good thing, which is why most professional reviewers have not commented extensively on it's pre-amp function. B) A Chord Mains Cable really created a blacker background and better sounstage, with more realistic timbres in the mids. This is cool, and shows that the amp can be upgraded for a reasonable amount in my opinion.
 
So, I'm still looking forward to trying this amp with a few more headphone and source combinations to see how much we can squeeze out of it, and perhaps get it up to at least four and half stars. (I'm probably always going to think it was a bit much to pay for a headphone amp! But I'm cheap sometimes.) 

I'm writing this because I feel the need to defend this little giant-killer. 
 
There are a lot of negative reviews, and while I am not on the "Schiit-wagon" at all, or a fanboy of their products, I now have a better idea of what kind of amps are out there, after owning this for two years and having upgraded dramatically to the Lehmann Rhinelander (around $500) and then the Lehmann BCL Linear (around $1200 or more). Guys who have been into hifi for years are shocked when they hear I have a headphone amp that costs $100 and basically reflects the source with no extra frills.
 
First of all, there is a sense in which no, I would not recommend this amp for IEMs, particularly super low-impedance Shures (I have the 425 which has 18 Ohms!). They have to be listened to very loud before the channels are balanced. BUT, and this is a big BUT, most solid-state headphone amps without a gain switch are going to give you the same problem of low volume channel imbalance (including the Lehmann Rhinelander which is 5X the price), and it is still probably better than a headphone output on a CD player or home receiver, so it may be worth it for that. Otherwise, if you listen to iPods or iPhones or better DAP sources you are probably not going to gain much through your IEMs. (This problem may be less dramatic with the new Schiit Magni 2, which I have not tried yet, but is reported to have a gain switch.)
 
Efficient headphones like Grados likewise, don't seem to benefit that much, but the Magni does add significant sub-bass as long as the extra high-range energy doesn't bother you.
 
I do not have the accompanying Modi and have generally used HRT products for my USB output sources. Unfortunately, from my Lenovo laptop, I often got a very high noise floor when listening to Beyerdynamic phones, which are notoriously sensitive to that. 
 
With dynamic phones that have over say 70 Ohms, and from what I've read, efficient Orthos, the extra power does only good things for headphones over a regular headphone jack. Assuming you have a decent source and cables, such as a non-portable CD player or relatively good DAC, you are going to find the Magni a fantastic value. Even if it's just a stop along the path of searching for a better amp, I think this is a good value as a very punchy, fast, dynamic, and powerful, headphone amp. 
 
Every amp has a sound, and this is probably just a bit on the V-shaped side of things, with slightly cooler mids and warmer lows and highs. This is well-known by now, which is why I had to get rid of my Beyer 990 Pros, but on the other hand, my more balanced sounding Beyer DT 150s sound fine.
 
Now that I have tried better amps, I realize that power filtering is really vital for sensitive headphones, and for some reason the Magni does not seem to have a very good soundstage, but this could be less of a problem with headphones that are not really oriented towards that direction. 
 
After years of IEMs and dynamic headphones, I think I'm going to finally go for some Orthos, and I'm definitely keeping my Magni. I highly suspect that with a decent CD player (even an older used Marantz, Sony, or Denon, Phillips, etc.) or a record player with some sort of clean phono stage and the Magni, plus any number of mid-range impedance dynamic phones (80-300 Ohms) that are relatively dark in signature, one of the many efficient Orthos like the Hifiman line or a modded Fostex, I'm going to be very happy (even though I already have a much better amp). 
 
With the Magni you can find a good used source and spend your money on getting the best over-ear headphones you can afford. Then, as you save, upgrade to a better amp, or not. (I do not, however, recommend the Magni with a cheap USB powered DAC that will leak in USB power noise!) 
 
Put it this way: if you want to listen to MP3s all day, just continue with your DAP and IEMs or one of the zillion types of decent efficient headphones out there that don't really need an amp. But if you want to enter the world of hifi music, you will need to upgrade to a 'real' source at the least, and these may not have headphone jacks. So, here's where the Magni comes in handy at the right price.

This is a good amplifier in my limited experience, *but*, it might not better than some of the leading solid-state competitors in this price range ($500-600). I haven't compared it too, for instance, Graham Slee, Burson Soloist SE, or Heed's Headamp. Also, you may want to think about tube amps in this price range, or seriously consider something from Audio-GD or Vioelectric. 
 
I haven't got a lot of experience with components, so I'll explain what I have tested:
 
Headphones: 
Beyerdynamic DT990 Pro
Beyerdynamic DT150
Shure 425
Grado 80 (old version)
 
Sources:
Fiio e10 (using line out)
Denon CD Player DCD 860 (mid-1990s)
HRT Music Streamer HD (USB DAC)
 
Interconnects
Chord (on CD player and DAC)
 
Lows are very punchy and tight, so high quality in my opinion, though one might ask for more quantity for some genres. 
Mids are detailed and rich.
Highs are detailed and clear as a bell. Well controlled and fast, so they are generally not fatiguing, unless your source and/or your headphones are fatiguing. If that is the case, this amp will not compensate for those problems.
 
I advise a well-balanced source and headphone, but if you have to go one way, I would tend to recommend a warmer source with generous bass, and darker headphones.
 
This amp doesn't ooze current like some out there (I think it may only have 100mw at 300 ohm, which is fine for the efficient German brands, but not for orthos), so it's fine for the headphones I've used. Output impedance is 5 Ohm, so best with headphones starting at about 32 ohm and going up, so while I can listen with my Shure 425 earphones, I wouldn't recommend this amp for them because of the high noise floor.
 
I haven't changed the gain, but it does have a high gain jumper. The pre-amp option isn't very good, in my experience. Maybe someone more tech-savvy could get it to work better.
 
A curious thing is that with my Denon CD player, there was very little difference between the headphone jack of that CD player, which is quite good, and the Rhinelander. The Rhinelander actually sounded a bit worse, in some ways, with a dryer mid-range. This is an old CD player, but it sounds very, very, good relative to it's current market value (about $100). The space between instruments is very wide, even if the actual presentation thus becomes a tiny bit unrealistic. The mids are good, and the lows are rich. However, the highs are not very realistic or accurate, all of a sudden becoming slightly grainy and tizzy. The Rhinelander doesn't really know how to handle this combination very well. It seems to prefer a lot of high-end detail to shine. All of that said: if you're really on a budget and listen to CDs, you may want to look for an old Denon or Marantz player that has a headphone jack: they're quiet and very good. However, if you listen from a computer, you probably do want separate DAC and headphone amp components.
 
So, when hooked up to the Music Streamer HD it sounds amazing: punchy detailed and full. Also, it sounds much better than my vintage Onkyo TX-330's headphone jack. Though the vintage receiver adds even more body to bass and mids, it's all fat and very little muscle. I was relieved to go back to the Rhinelander. 
 
This little guy is a muscle machine. Kind of like a Jack Russel Terrior of amps. Cute. Fast. Smart. Lean and strong. 
 
Or if you prefer a different metaphor: it's a clean and crisp traditional German lager.

These are great headphones I was turned onto from the forum. 
 
On this forum you have to assess who is enthusiastic about what. Generally, there is a difference between enthusiasm from novices and that of veterans. The novices were excited about my Beyerdynamic 990 Pro, 250 Ohm. But the veterans were on to these DT 150 headphones. The sound is so much more mature, even, musical and rich on the DT 150s.
 
I will say, I think the sub-bass was a little better on the 990 Pros, but that's about it. Most acoustic music sounds better with the DT150s.
 
They also sound fantastic whether plugged into an iphone/ipod, and even more fantastic when put into a multi-thousand dollar system.
 
I'm currently using the HRT HD and the Lehmann Rhinelander, with Chord interconnects. A great sound, with a lot of punch and detail. I may go up to the Linear Black Cube just to squeeze another few drops out of these bad babies.
 
Loven' em. But I will say, it's the winter now and I'm not lookin forward to summer in the hot and humid place I live. These vinyl pads do get hot, and it's not nice to wear them when you're sweaty. (Are any over-ear headphones really?)

Background: I've had these for over five months and put at least 200 hours on them.  They still have a lot of energy in the highs, yes, and taming them is an essential part in enjoying them. They are super comfy and super well-built, with lots of real metal. 
 
Important Note of Preference Regarding Gear: All parts of the chain matter here: dac, amp, interconnects and the recording itself. Sibilance appears in vocals on MP3 tracks, but not with WAV files and above usually, especially of your DAC can use an ASIO filter. (I know it's controversial scientifically, but these phones seem to highlight the harshness of other output modes to my ears).  They can sound great out of good CD player's headphone jack, as in the old Denon and Marantz ones especially. They are also good with integrated receivers, which have analogue EQing possibilities.
 
Strengths: These are very rhythmic headphones with raised lows and highs that can be too much for many genres but means they shine on nearly all good acoustic recordings, including choral music and, up-tempo classical, bluegrass, and world or folk music that is very acoustic oriented.
 
They do amazingly well on orchestral works, and can make sense of very complex music.
 
With the right amp and source, the mids are very good, in my opinion.
 
Highs might be seen as a strength to some, because they give some air and realism to violins, female voices, and this is a plus on good recordings in my opinion.
 
In addition, solo music that is meant to represent real instruments like classical guitar, piano, harp, cello and other instruments that need a bit of beef in the lower section sound more realistic than any other headphones I've tried. They also add clarity to spoken or sung lyrics, but the mix doesn't bring vocals to the front like some phones. 
 
They are fantastic for movies and gaming, especially if you watch actual DVDs.
 
Weaknesses: The problem is that the highs are truly exaggerated right at the point that is generally higher than the human voice and most acoustic instruments. So in most systems, you may have trouble with many rock, pop, and electronic genres. Even hip-hop can be annoying because of sibilant vocals, making guys sound like they have a lisp, particularly on less than amazing recordings, which is 90% of all hip-hop, anyway. I have listened to Green Day, Neil Young, Nirvana, the Rolling Stones, and some others (not a huge rock collector, actually), and while I particularly like what these phones do to punk rock, ska, and complex songs, they generally get fatiguing very fast. Most of these genres boost the treble anyway to compensate for the poor systems found in cars and the average home stereo or stock IEMs. 
 
I found that even with higher resolution tracks, such as Peter Tosh's "Equal Rights" 24/96, by HDTracks, the cymbals really get on my nerves very quickly. Strangely, Wav files from 16/44 Reggae recordings typically bother me less. It could be with certain mainstream "remasterings" of Bob Marley and Peter Tosh, they really cook the treble to sound better on inferior speakers. Lesser known albums such as Culture's "Harder than the Rest" sound great with lots of layers of percussion, but still grate after a while. 
 
I personally do not find them perfect for EDM. The highs get annoying very quickly with this genre, even though the spacing, imaging, layering, and bass is very good. Probably you would want to go with one of the Hifiman orthos if this is your gig. I liked Daft Punk's Random Access Memories album almost better with my Sure 425s, due to their speed, flat bass, smooth mids, and rolled off highs.
 
Older jazz recordings can also come up short under the microscope of these cans, even if the overall enjoyment might be there. They are just too unforgiving of the slightest distortion (at least when heard through my solid state Lehmann Rhinelander, or the Schiit Magni (major earache)).  Newer jazz recordings are generally good with these phones, but it depends on how many electronic instruments are being used, and how they mix the percussion. If it's too hot, you're probably going to find these a bit tiring for some jazz recordings as well.
 
I find myself having to adjust the volume sometimes, even for acoustic music, due to the imbalance of the highs. For this, as the 6moons review noted with the DT880 600ohm (which they also recommend for jazz and classical), you need a good volume pot that can be adjusted ever-so-slightly within tracks, between tracks, and certainly between recordings.
 
They also don't do justice to many older recordings, due to the exaggeration of flaws, pops, clicks, and tape hiss. This is very unfortunate for me, and one of the reasons I'm thinking of moving on to a more forgiving headphone.
 
***Some say you can EQ these phones to have smoother highs, but my experience is that EQing doesn't help much. I don't think I'm the majority opinion here, but I feel that the acoustic features of this phone are hard to override without getting negative side effects such as distortion.
 
Comparisons and a note on Value:
 
The absolutely crazy thing is that I bought them for $150, and I bought my Fiio E10 for around $60 a couple of years ago.  With literally nothing else, you have leapfrogged over the vast majority of low-fi and low-mid-fi set-ups costing three times this much. Not only that, you would have to spend many times as much to get a full room set up that sounds even close to as good. No I use the HRT HD and the Lehmann Rhinelander with Chord interconnects. It sounds very, very good, but a bit too much energy in the highs for most non-acoustic music. The good thing is that I'm not worried that I'm missing any details. I'm sure some very pricey cans, such as the T1 or HD800 can pull up more micro-details, but honestly, more detail is not the problem here, but rather the slightly less-than-smooth sound that results from the extended highs.
 
Nothing else that I know of in the $150-250 range will clearly beat these in terms of accuracy, meat in the low-mids, and ability to scale up to a very serious mid-fi sound system.

First of all, I don't think we should be comparing these to anything else but in ear monitors (IEMs), and generally in the price range.  However, my only experience is with Sure SE 110s that I used for over 5 years.  Those were about $99, and I got the 425s for about $270.  I've now had them for about four months, and have gone through the slightly disappointed stage which seems to occur for a lot of people with a lot of headphones.  No matter how much you read before purchasing, it still takes time for your brain to get used to a new sound signature.  These really blow away the 110s in every way.  They have energy, speed, focus and simply amazing imaging.  I can't say the last part enough.  It's not soundstage, per se, (because there's not much air) but rather the ability of these phones not to get bogged down with complexity.  They just keep up, keep up, and keep up.
 
So, as others have noted, the bass tones are all there, and in fact I've noticed these IEMs are great for rhythmic music based mainly on acoustic instruments and vocals ranging from jazz to funk, reggae, to many kinds of "world" music.  Also baroque music and small to medium sized classical ensembles sound great.  This is because they really neatly lay out the different textures well.  Marvin Gaye's remastered "What's Going On" for instance, is fun to hear.  On the other hand, music that tends to be a bit dull and lacking in rhythmic or tonal variety to begin with might sound even more dull on these.  
 
Moreover, the mids are great: smooth, musical and accurate whether you're listening to vocals or basically any other acoustic instrument.  After all most acoustic music takes place in the mid-ranges.  The main thing lacking is the sparkle on the high end, but even there, I would hesitate to say they have no highs.  They can make some music sound pleasant though, where my Grado 80s (old version) become grating. 
 
I think technically they lack some treble, but the roll off is nothing like the 110s, and since the sound is going directly to your eardrum, the dampening of piercing highs is not such a bad thing.
 
If you're really into genres that use a lot of what I call 'artificial' highs and lows, you would probably want to avoid these phones.  They are also not amazing for large-scale orchestral work, and can at times be a bit boring for non-acoustic rock.  I don't listen to any of these genres much, except for some hip-hop** and orchestral music.  In both cases, you still have a great musical experience, but not to the level of good $300 over-ear phones, that's for sure. 
 
They sound significantly better, as can be expected, with lossless and high bit rate tracks, but are very sensitive, and so don't do well straight out of the computer jack, even making amping a bit difficult.  I use the Fiio e10, which is fine when listening at moderate or high volumes, not necessarily low volume though, as I get buzzing from the computer (Lenovo thinkpad) when it's charging.  I also have a Music streamer ii dac and schiit magni amp, which do open them up a bit, and especially on high bit rate tracks show off what these IEMs can do.  They are elegant and have excellent instrument imaging and separation, for example.  This combo also brightens them up, which is good.  You still can't listen on very low volume, however, as you'll probably get some channel imbalance.  
 
They are a bit annoying with phone calls, because as they don't have phone capabilities (without a cord upgrade), you have to take them out and reinsert them each time you get a call.  It can affect your relationships negatively if every time you answer the phone you are annoyed--both that you had to stop listening to the music, and because it takes a few seconds and some effort to put them in and remove them each time.  
 
Conclusion: These are fine for acoustic music, which in my opinion, is essentially what music should be.  The imaging might make them good for electronic music, too, but bass heads won't like them.  But the advantage of these is the external noise attenuation, portability, and sensitivity, making them great for portable devices.  With these on my daily bus ride in Tel Aviv, I can block out virtually everything except the vibration that comes up through the seat from the bus itself.  Plus, in a hot climate you don't want anything on your ears when you're on the go, not to mention that some phones are just plain heavy, and as a backpack carrier, I also don't need extra weight.  I briefly thought about selling these and getting Sennheiser 25ii headphones, but backed out due to my own portability concerns.  Probably a toss up, with the se425s coming out a bit on top with respect to mids, imaging and balance, but 25iis, to my experience, having more bass impact (but not more bass tonality) and better high extension.
 
**If you actually want to hear the words of the hip-hop, these can help you with that, as they're quite detailed and can show you where there are several voices going on at the same time.   They just won't give you anything like a club feeling because of the lack of air in the bass.