This is a very long overdo review.  I've owned these for about 15 months, give or take, and use these about 8-12 hours a day.  I've toyed with the various filters, and have tweaked them a bit, but in the end, after tweaking, these are absolutely the best things I've ever heard sound through.  I spent the better part of owning these swapping comply tips every few days, swapping filters back and fourth, and tried a ton of various sources.  Keep all of this in mind while reading this.
 
I'm going to cut right to the chase: if you own these, then there are two things that you MUST do to actually appreciate these.  First, and foremost, you need to put the blue filters in, and you need to pull out the crappy foam that inhabits the tubes within the filters.  Before doing this, you're appreciating mediocrity, but afterwards you're rewarded with something equivalent to a Ferrari in the headphone world. The soundstage is easily ten times better, bass is significantly cleaner and more lush, and the highs twinkle to an unbelievable extent.  The before and after foam removal is like comparing $200 headphones to $1500 headphones.  I genuinely feel as though this foam unneedingly cripples the audio nirvana.  Secondly, you need to grab yourself an xacto knife, and you'll want to splice out the metal paperclip that promotes the memory loop.... without this metal, the comfort skyrockets.  This is assuming that you have the factory cable, of course.
 
I typically break down music during any reviews or comments, and claim a headphone is fantastic during certain parts, and crippled during others depending on the music.  This is different.  Anything that you're going to listen to, after the filter mod will push your ears to their apex.  When getting newer, and higher-end headphones/IEMs, you're rewarded with a smirking face the first few weeks you put them on until you become accustomed to them, but these provoke the same sensation every single time you put them in and press "play."
 
Obviously, I seem biased, but what can I say?  After the foam mod, they actually fight to make sure I'll never want anything better.  They make their point very well, and honestly make me appreciate not only new music, but my previous collection as though I've never heard it before.  Granted, if you're a potential buyer, you're going to think to yourself "I'm paying $1,000 for some headphones, why should I have to tweak them?" and you're right to say that.  Out of the box, they honestly sound very good, but they didn't set them self apart from other extremely high-end headphones/IEMs.  This isn't to say that you won't be extremely satisfied with their default output by any means.  I just want to get it out as soon as possible: if you aren't planning on removing that crippling foam from the blue filters, then I would say just avoid these completely.  I also want to bring up the subject of tip placement, and why these stand apart from other models/brands... the metal tip tube is fantastic, and coming from many other IEMs from various price tiers, I will say that they are significantly more modular as far as tips are concerned.  I have always advocated Comply tips, but in the end you'll have to swap them out at the most weekly, and without that swap you're faced with a dilema of laziness and SQ.  I've always appreciated rubbery tips, but who am I kidding?  They don't promote the same type of seal and isolation that Comply or similar tips provide.  The metal tubing on the 846s, with the barb system allows you to slide the tips as needed for the perfect fit.  I hated dealing with Comply tips, but I obviously couldn't use anything else to get remotely close to the fidelity that they provided via their seal.  With the metal tubes on the 846s, you can drop down a size on the rubber tips that fit you, and slide them down the tube so it pushes further into your canal to provide the same perfect fit that you'd expect from other types of tips (like Comply or tree tips).  It's nice to get that perfect fit without stressing over tips.
 
Okay, so what really shines with these?  Complex layers are separated entirely through them.  Vocals, both male and female, sound blissful and seductive!  I don't typically listen to vocal music, but when it happens, I feel so blissful that I could simply pop!  Bass riffs?  You got it!  Bass slaps and riffs grind down to your ears until it provokes a grin.  I typically listen to various electronic music, such as electro-pop, trance, chillout....etc, and the layers are so luscious and organic that I borderline can't stand it without being happy.
 
I am from the 80's, so 90's grunge is very nostalgic to me.  After owning some UE Triple.fis, I assumed that fidelity doesn't work well with heavily distorted guitars, but these aren't fatiguing like the UEs, and instead creates extreme fidelity between each pick during a guitar riff... perhaps the triple.fis were just to "bright?"  I'm not sure, but I'm very happy to admit that I don't have the same problems with the 846s.
 
To end this review, I've read a rainbow of good and bad reviews of the 846s.  I definitely assume that all bad reviews are from people that didn't do the foam filter mod.  It's easily night and day.  Listening to them with some lossless and a great source makes my old flagship headphones feel like VHS, and these feel like bluray.  I honestly doubt that not only will I never buy another pair of headphones or IEMs, but I doubt I'd ever want to.  They're that perfect in every single way.  Sure, there's IEMs with more BA drivers, and some that are custom, but what would I care?  These are quite simply, perfect in every way when it comes to sound.

I started this review with the intention of pointing out the things I don't like about it, but man, there really isn't much I don't love about these.
 
Out of the box they sound very thin and anemic, but still good transparency.  I took them off immediately, and put them in the included case, then turned the volume up pretty high for 30 minutes.  When I returned to check on the status of them, I was stunned.  Coming from a pair of UE Triple Fis and some K701s, I knew it was a downgrade, but at the moment up putting them on after the 30 minutes..... I realized I shouldn't have assumed.  In a nutshell, they remind me A LOT of my old silver cabled K701s (after burn in of the 701s).  The transparency, the attack, the huge soundstage (seriously....they're in the K701 tier), and the coldness of the signature.  I read every review I could, and the word "warm" kept popping up, but I have to disagree.  Imagine the very cold K701s with some of the most lush liquidy bass you can imagine, and you'll have a good idea of what to expect from the P7s.  These are obviously just my thoughts with my past setups, and is extremely opinion, so from this point forward I'll try to touch every aspect of the phones, and then I'll do some music testing.
 
Build Quality
 
Okay, I'm not going to try to use sexy words that compare them to high-end automobiles.  These things are tanks.  They're mid-weight.  The leather smells like high-end by itself.  I was concerned about the rod design, as I've never owned a pair of Bowers and Wilkins before, and I didn't know what to expect, but they're rock solid.  The replaceable ear-cups are a great touch, and I don't have a doubt that I'll probably own these for a few decades or more as long as I can get ear-cups and cables.  There's honestly not much to say about this area, because you instantly realize why they are so costly from the very second you slide the lid off of the box, and it's not just an initial reaction.... it's one that never goes away.
 
Comfort
 
It takes a few minutes to get the comfort just perfect, but once you get it all situated it's like putting on a marshmallow helmet.  The headband will hurt if the sizing isn't adjusted perfectly, and I mean perfectly... an inch off throws everything off.  Undoubtedly, if you're in the market for these, you've read some reviews talk about the excessive clamping; out of the box they were way too strong and hurt, but I stretched them out in a realistic manner (what it takes to put them on plus another three or four inches), and now they remind me of the Sennheiser 5xx series.  I've worn these for 15 hours at a time, and I simply forget they're even on my head, but granted I don't have the hugest head around (avoid these if you're a Stewie Griffin).  I assumed that the ear-cups would be too warm after hours, but so far no issue (no sweat or anything).
 
I just want to add this for anyone that may wonder: I use my headphones to watch TV while I'm trying to go to sleep.  I lay down on my pillow with these on, and they don't do anything unusual like most OTE headphones do. 
 
Treble
 
These really can sparkle.  They aren't harsh though, and I can honestly say they're within my treble sweet spot, because they don't fatigue, nor are they harsh, but they are right there at almost being able to do that.  In a nutshell, they treble is about perfect for the music I listen to, and they handle distorted guitar solos well if you're into that, while the K701s were a bit too harsh for extended listening of this genre.  I do use these to watch TV too, and the treble is good enough that I can hear the variances in the audio tracks during a single scene... like a CRT sound when one character has dialogue, and the character responding doesn't have it.  Cymbals, snares, and reverb audio is extremely clear.  Be warned, however, these SLAUGHTER compressed audio, and it almost makes it unbearable.
 
Mids
 
The mids are very intimate compared to the rest of the spectrum.  Male vocals are probably some of the best I've ever heard through any headphone, but female vocals are very good as well (though not as good).  90% of the music I listen to is electronic, and any Moog synth mid-basses are stunning (like deadmau5's stuff).  While I think the mids sound wonderful, they don't sound top in class to me, as in I can't point out any problems, but also can't point out anything unusually good.  I don't feel that the mids are recessed though, so don't take what I'm saying as that.
 
Bass
 
I think these headphones REALLY shine in this area, so to speak.  I haven't heard a headphone handle the other frequencies so well while genuinely impressing me within it's bass range.  Like I said, I do listen to mostly electronic, but I'm an audiophile electronica junkie... not a V-Moda bass junkie.  These things are extremely accurate with their bass, and have been using them for some music production (electro-house and progressive), and they really do keep up with what I KNOW is going on behind the scenes.  The most impressive part about all of this is how low these things go... I mean they love to wrap your mind in a lush liquidy layer of sub bass (~30-40hz) while still extremely accurate with the kicks, snares, and synth riffs I'm throwing at it.  As a test, I threw about 40 instruments together in a DAW, and I could pick out every single instrument while I had a steady bass pad going (FYI, the k701s would distort significantly faster than these... but this is borderline abuse for any non-BA speaker).
 
Now I also play acoustic instruments, and have a bunch of acoustic recordings and music.  I down-tune my acoustic guitar to D standard, and the P7s play it as naturally as I play it.  I'm adding this because a lot of headphones that love bass will bloat this sound out, while these don't as far as I can tell.
 
Music Samples (all are 24-bit 96kHz vinyl rips)
 
Carbon Based Lifeforms - Frog
 
There's a DX7 synth during the intro, and man it sparkles.  Xylophones quickly accompany the DX7 within the first 30 seconds or so, and the xylophones dances around in mid-air.  The entire track has a VERY warm pad that makes you all gooey inside, and it's almost bazaar how it stacks into the background... almost like it's been there since before the song started, and is a very natural and organic sound.  This isn't a bassy track, in case you aren't familiar with it, and don't want to go hear it, but there's a very great mid-bass that goes on throughout the track, and I physically feel this on my ears (the presence of this is very powerful IMO). 
 
 
M83 - Midnight City
 
Okay, this song is very fun, and it's very synthetic sounding.  The stabs are very detailed during the beginning, and the Moog bass is delicate.  The layer of low bass that creeps in is very hot and just lovely, but it doesn't overwhelm the other instruments at all.  The vocals are FANTASTIC... absolutely my favorite part paired with the P7s.... very smooth and natural.  The snares throughout the track are almost overly detailed, and I hear everything about them (I'm not sure if the headphones are emphasizing on this range, or if the song does).  There's a looping sound towards the last quarter of the song that sounds like wire sticks hitting a symbol... I have never heard this part before, and it's a great example of how detailed the highs are on these.
 
 
Daft Punk - Lose Yourself to Dance
 
I'm going to go ahead and say that I've never heard drum kicks so amazing on any other headphone of any price-range ever.  Seriously, I'm not sure what it is about these drums, but it's almost like they were made for the P7.  I know drumming equipment well, and these sound very real and right in front of me (and a little to the left ).  The palmed guitar strums are very sparkly, and as a guitarist I can hear every detail of what's going on with that.  The vocals are very good, but not as good as Midnight City, but it still sounds amazing.  I've never been the hugest fan of this song, but when paired with the P7s it becomes EXTREMELY fun.  Just when I had them pegged as cold and heartless it pulls something like this out.
 
 
Days of the New - Die Born
 
What can I say, I'm a sucker for acoustic.  This song is HUGE, organic, warm, and extraordinarily detailed.  Such fantastic recording in this song, and it is the highlight.  The guitars dancing around on a 3D plane all around you.  The vocals are very dynamic, and right in front of you (sounds like 10 feet in front of you).  The cymbals within this track are extremely detailed, yet light.  The distorted guitar comes in and is not dominating, but rather paired with the organic sound of the acoustic instruments like cheese is to wine.  This song is a bit hot towards the half-way point and all the way to the end, but even though the P7s are trebley at times, they aren't harsh during any of this.
 
 
Goldfrapp - Strict Machine
 
This song is has some of my favorite vocals in it, and it really proves me wrong about female vocals not sounding amazing with the P7s.  There's those dry airy vocals that demand to be heard, and you can't not analyze every single second of the vocal track.  This entire track is composed of square waves, and boy do the P7s perform.  Out of every song on this list, the award for hugest soundstage goes to Goldfrapp.  Some of the instruments sound like they're 20 feet away, some are right next to me (like the square waved bass), and there's a vast ocean of extremely intricate instruments dancing everywhere.  There's these bass riffs that play every measure, and they sound like they're developed in the distance, and rush towards you... which is something I've never experienced.  This song has so much stuff going on in it that it's like static of amazing sounds, and they're all covered up with a blanket of deep deep deep bass that you'd expect from only a sub-woofer.
 
 
Well, there you have it.  These are fantastic headphones, but I don't think I would say they're quite reference.  That isn't to say they aren't as detailed, but rather there's a little bit of color.  The mids are of good quality, but the highs and lows are AMAZING quality.  I would say these are the bassiest cold headphones I've heard, and that seems like a strange thing to say, but that's the impression that I'm left with.  I think that a few years ago, when you had the three kings: HD650, DT880, and the K701s, if you prefered the K701s, but wanted more bass and something a lot easier to drive, then you should give these a test drive.  If you loved the HD650s, then I doubt these would be for you unless you want something a bit less "dark".  I'm not going to say that the P7s belongs with those three headphones at all, but I'm using that as a reference since most people know of those, and we can't all go to a local hifi shop or apple store to give these a try.  The fidelity of these is top-notch, and I recently sold my triple fis; I got these with the assumption that I was stepping down in fidelity, but I was wrong to assume that, however they are a lot different (you simply can't compare a headphone to IEMs, and I'm not going to try).
 
 
 
 
 
UPDATE: So I love these headphones a lot...they're fun, they're built great, and I wanted to up their game some.  I decided to recable them since they have a modular cable design.  I ended up squeezing 18G copper into the cup without moddification, and without stressing the interior at all (I'm weird like that).  The soundstage exploded, and sound more open than any non-soundfield design I've ever heard (they actually remind me of the Sony PFR-V1's soundstage).  Most importantly, however, is the transparency! Anyway, if you own a pair of these, or are going to get a pair of these, just make sure you swap that cable out!