I was listening to music through the Peak yesterday.  And it was such a thrilling listening session.  However, I did something unusual: I stopped.
 
I hit the pause button after a particularly great song, and took off the headphones.  I held my breath for a moment, and closed my eyes.  I wanted to remember this moment.  Just in case it never sounded this good again.
 
"That was it," I said to myself.  The place I've been trying to find for a few years now.
Then, as I recalled the long and expensive journey that got me here, I became a little upset with myself.
 
I was upset with all the money I had spent on "audiophile headphone amps" which, as it turns out, left so much to be desired.  Then I thought to myself:  Without that journey, would I be able to appreciate just how good this amp truly is?  Probably not, I concluded.  But I suppose I'll never know for sure.
 
So, I came back to the Peak today, and began listening.  And, you know what?  The magic is still there.
The music is immediate, alluring, and crystyal clear.  It is exceptionally well defined, but never analytical.  I can follow each instrument, and locate it spacially.  From the moment of attack, all the way through its decay.  Transients are incredible, but the music never becomes shouty.  In fact, the control through busier passages is something of a revelation to me. 
I will admit that I hear something of an emphasis on the upper frequencies, as others have.  But I will go one further in saying that my ears have been listening to tube amps for a while now, and I'm fairly certain that there's simply more high frequency information here than I'm accustomed to.
 
In summation:  This amp is fantastic.  That's just all there is to it.
 
Update - May 2012:
I just purchased my second Peak/Volcano, giving me matching (sort of) work and home rigs.
And I am pleased to report that the standby and power lights on the Peak/Volcano now are matching yellow, in lieu of the green standby LED used in the first batch.  A subtle change, sure, but one that makes the Peak/Volcano a touch better looking when switched off. 

After reading some glowing reviews, I decided to give the HDP a go.  I needed a compact and hassle-free solution for my work rig, and one that could drive any headphones I decided to bring to work with me (including the difficult-to-dive 600 ohm beyerdynamic cans).
 
After receiving the HDP, I was impressed with its looks and build quality.  Even the packaging and branding were pretty sleek.
 
However, it was upon powering up the device when my impressions started to turn for the worse.  The HDP is, to my ears, a pretty nice source paired with a somewhat bright and not exceptionally powerful headphone amp.
 
The DAC section of the HDP is nice, but perhaps a bit on the analytical side.  It is detailed and wide open sounding.  The headphone amp section of the HDP, however, I found a bit bright for my tastes.
 
I found that using EQ to roll off the treble gave me the sound I was looking for, but the nagging feeling remained that I could find another amp to more suit my needs...
 
If I were using the HDP as a DAC only, I perhaps would have kept the unit, as it was a nice sounding source.  However, i bought it to perform both DAC and amp duties, and as such it was a slight disappointment for me. 
 
All this is not to say that different folks may not like the product.  But I, for one, found its sound signature was not exactly to my liking.  Functionally, and otherwise, it was more than fine.

Just throwing my two cents in... 
 
These headphones are terrific.  And having come from some of the older flagship headphones (K702, DT880, etc.), I can say these are clearly a more reference quality headphone.
 
The sound is revealing yet never harsh.  The treble response, to my ears, is sweet and very slightly rolled off.  It is part of the reason why these are not fatiguing headphones.  And the midrange?  Boy oh boy...  It's the most resolving midrange I've ever heard.  Voices and instruments not only sound fantastic, they sound more real than with other headphones I've heard.  And by some measure, I might add.
 
My only nit to pick with the T1 is a slightly difficult one to describe, and it pertains to the bass.  (And I'm perhaps in the minority on this as well.)  While the headphones have neutral bass quantity, and nice bass impact, I find that the bass recedes slightly during complex sections of some music.  It's not terribly objectionable, but something that my ears pick up on from time to time, and keep these headphones from reaching a pure 5-star audio quality rating, for me. 
 
Also, to be fair, there is a shadow of a chance that my amplification is not up to the task of driving these cans.  While they are rated at 600 ohms average, the impedance at ~100Hz is off the charts, and is reported to be somewhere near 1300 ohms.  Wow...  So, in order to get bass thump, you really need beefy amplification.  I will revise my assessment of this nit-picky problem if I come across an amp that helps resolve the receding bass...  Otherwise, it is, like I said, a minor quibble that I can live with, in an otherwise stellar set of cans.  Terrific in fact...
 
And, if the price tag were a little lower, it would be easy to recommend the T1 to anyone and everyone.
 
Edit:  My quibble with the disappearing bass seems to be tied to amplification after all.  I have just received the DNA Sonett amp, and the bass throught the T1 seems more fleshed out, and seems to sustain through busier sections of music. 

Just trying to pull the average up.
can't believe someone would give reference headphones of this quality a 1.5 star rating overall.
but, that, i suppose is the love/hate nature of the k701/k702.
 
personally, i feel they are wonderful headphones.  and, at $260 street price at time of this posting, you'd be hard pressed to find a better sounding pair for the money.
 
they are a little light in bass impact, and a little strong in the upper mid (female vocal range).  harmony vocals can get a little shouty sometimes. 
 
but, what they do well is render acoustic instruments with startling realism, and produce a spacious sound that is perfect for listening to concert hall recordings. 
 
ideal headphones for unplugged recordings, or live jazz, or classical, etc.
 
not ideal for hard rock, electronica, etc.

the woo wa6 is the single-chassis smaller sibling of the wa6se.  but don't think for a moment that listening to the wa6 you'll be "slumming it".  not at all.  because, the wa6 is, quite simply, a wonderful amp. 
 
from the tank-like build quality, to the sexy anodized brushed aluminum finish, to the amp topology, everything is top notch.  and the sound is great in all areas.
 
frankly, i can't believe i'm the first to jot down a review of this popular amp.  i'm sure others will follow.
 

The caliente is a really solid entry in its price range for a solid state headphone amp.
I would venture to say it performs a bit above its station, at $399 Canadian.
 
The sound is smooth yet powerful, and the volume knob is just perfectly tuned with the amp.  There's plenty of power on supply, and the amp drives 600 ohm cans without stress.
 
The Caliente is the "warm" sounding sister amp to the standard K.I.C.A.S. model (stands for "keep it clean and simple").  And i can detect just a hint more warmth and bass impact than perhaps would be there with a neutral amp.  And the sound is all the better for it in my book.
 
I'm really pleased with this amp, and would recommend it to anyone seeking a slightly warm sounding amp at this price point.
 

i bought the ef2a as a dac/amp for my work rig, hoping it would represent a nice step up from the portable dac/amp i was using at the time - the ibasso d4.
 
immediately out of the box, i was struck by its looks.  its an attractive little unit.
 
however, after a couple weeks of use, the dac/amp really started revealing its weaknesses. 
 
first, the dac section of this unit is nothing special.  comparing it with the d4, the d4 is superior, providing more instrument separation, more clarity, and less congestion in the lower mids and upper bass. 
 
and so i kept the d4 in the chain, and decided to use the ef2a as an amp only.  but, the amp section has problems as well.  the first of these is an unruly amount of gain.  so much gain, in fact, that the volume knob is rendered pretty much useless.  
 
by way of explanation:  let's say i'm using grado cans.  and i'm listening to mp3 using foobar.  i have my computer volume and foobar's volume set to max, as i should.  i turn on the ef2a, and begin turning up the volume until i am comfortable with the volume level.  the problem is, this volume level doesn't exist on the ef2a.  because comfortable listening level is blown past easily in the region where channel imbalance is still an issue on the volume knob.  and when i say channel imbalance, i mean it.  no signal in one ear until 9 o'clock on the dial.  and then suddenly the channels balance out, with raging volume at 10 o'clock on the dial.
 
so, what does this leave the listener to do other than lower the volume of the source?  and, as computer-savvy head-fiers know, this doesn't do any favors for the sound quality.
 
and that is a problem with the function of the amp.  i haven't gotten to the sound yet.  it has a few problems as well.  however, in fairness i haven't tube rolled the unit, so i can't say if some of the weaknesses of the amp can't be overcome with a little more money and effort.  (personally, i didn't feel tuberolling a $170 amp  would be fruitful.)
 
mid-bass, all the way through lower mids seem somewhat closed-in and congested, and the treble seems a bit brighter than neutral to my ears.
 
and, the amp doesn't really have a lot of power.  in fact, it's frustrating to me that it tries to disguise its lack of power with gobs of gain.  this makes for a frustrating listening experience, as i'm constantly toying with the volume level in foobar and on the amp to reach a listening level that doesn't seem fatiguing, and doesn't have that strident sound of cans being underpowered. 
 
in fairness, the ef2a sounds pretty nice with grado sr125 and sr225.  this is, in my opinion, a happy accident of headphone synergy more than anything.  so, if you're willing to cut the volume on your source by half, then the ef2a does play nice with those cans.
 
and so, overall, it really sounds as if i'm bashing the ef2a.  but, i can't help but expect a little bit more from it.
 
also, did i mention that the rca inputs are reversed as well?  yep, when feeding the amp from another source, i have to plug in the rca's in backwards to correct this.  however, that's just a problem with my particular unit.  but it's indicative, i think, of what you get when you go this route with hifiman...
 
a route i won't be taking again.

The Fischer Audio FA-004 is an enigma wrapped in a riddle, wrapped in an enigma.
 
This Russian headphone has just garnered attention recently in the Western world.  And, with a selling price of $67 at Bugden Audio, it should hopefully take off as a really viable option for someone looking for a portable, nice sounding set of closed cans at a budget price.
 
So, what does this cheapy set of cans sound like?  Well, that's the enigma.  They are smallish, barely-circumaural cans.  And, as a result, they simply aren't able to convey the nuances of more expensive and more full-size headphones.  But what they do offer is a sound signature that is tuned to give you a lot of enjoyment.  A sound signature that does a pretty good job of disguising its own shortcomings.  Which are a significant roll-off in the lowest frequencies, and a slightly thin and less than realistic-sounding treble.
 
These headphones are fairly forward in the mids, and fortunately they have pretty nice sounding mids as well.  They have a slight mid-bass bump to compensate for not reaching those lower frequencies mentioned.  Using the "Heartbeat" track from the headphone test album "Open Your Ears", the final low frequency heartbeats are barely there, with the headhpones giving only the initial impact of the bass note, and hinting at its sustain and decay. 
 
Another enigma about these FA-004 is that, for a closed headphone, they feel surprisingly open.  They don't have strong clamping force, and don't create a strong seal when worn.  This combined with their light weight makes for a really comfortable headphone that doesn't sound closed off, and doesn't create tons of pressure within the ears.
 
The imaging and soundstage are both pretty average, but certainly better than any other sub-$100 headphone I've ever heard.
 
At $67 plus shipping, these really do represent an astonishing value.  They've become the headphones in my work rig, and I'm enjoying them immensely.  They're such a  value that they have piqued my interest in their bigger siblings, the FA-002 and FA-003...

wow!  am i really the first to write a review for the venerable portapro?? 
 
the portapro is an age old design, but has held up over the years, despite newer portable cans coming along.  and perhaps it is the very age of the design that keeps people coming back to it.  what with its alnico magnets and paper diaphrams.  and it's weird retro look.  and its mega fun sound!!!
 
yep, the sound is fun.  the portapros have a nice, punchy sound.  and great bass for a small supra-aural set of phones.  they are great for pop, rock, hip/hop and the like.
 
the comfort is okay.  the temple pads are supposed to releive some of the pressure of the ear pads, but i find that they don't really do this much for me.  also, the adjusting metal headband is prone to catch your hair and can hurt when it does this.
 
overall, for $30 to $40 street price, these are such a wonderful product, and very enthusastically endorsed by me.  (as i'm sure they will be by others here.)
 
a classic!  and with reason!!

the hd595 were my first nice set of nice cans.
and, as a result they will always have a special place in my heart.
 
at $190 street price (as of this review), though, there are just so many other cans out there right now that will give you much more excitement for your dollar.
thing is, the hd595 borders on boring, in my opinion.
 
yes, they are fairly neutral, have a pretty respectable amount of detail retrieval, and are very comfortable to wear and listen to for hours.  sure, part of the whole "forget you are wearing them" thing is the comfort.  but another reason is the somewhat flat sound signature.
 
overall, i liked them fairly well.  but, quickly found that scaling up from the $190 hd595 to a $230 DT990 yielded a substantial improvement in sound quality.  i'm sorry to say that, in hindsight, this makes the 595 look a little overpriced.

the m50 was released a few years ago by audio technica as the flagship of their "m" studio monitor line. 
and, like all audio technicas, it is colored in one way or another.
this particular headphone definitely has more bass than a strictly neutral set of cans should.  but, the bass is quite impressive for a phone of this caliber and price range.  using the "heartbeat" test track from the "Open Your Ears" headphone test album, the m50 produces more low frequency rumble than any headhpones i own or have heard.  (bear in mind i have not heard denons or ultrasones).
 
the general sound is warm.  the headphones are colored forward in the mids such that the bass does not step on them too much.  this creates a fairly engaging sound signature.  the highs are recessed just a tad, fortunately, because they are just a tad grainy as well.
 
the imaging and soundstage are pretty respectable for closed cans.  but, the soundstage doesn't really "take off" until they are amped and turned up pretty loud.
 
when listening to these extensively, you get used to the presentation, and the one drawback that reamins is the mid-bass is just a tad unresolving.  i wouldn't go so far as to call the mid-bass "sloppy", but it doesn't have the attack and decay of higher end headphones.
 
overall, the m50 performs very well for a headphone that you can buy for around $100. 
it's biggest drawbacks really don't come so much in the form of sound quality as they do in the region of comfort.  As the clamping force is a bit much, and prevents me from wearing them longer than a couple hours at a time.
 
Also, as a result of them being closed cans with a tight seal, your ears will require quite a bit of adjustment when putting these on.  after a few minutes, the pressure will equalize, but at first it feels like your ears need to pop.
 
as for audio quality.  i struggled between giving these 3.5 stars and 4 stars.  but, i have to be honest here.  if 3.5 stars is "very good" and 4 stars is "excellent", then 3.5 stars it must be.  they sound very good to my ears.
 
edit:  after a couple months constant use, it seems to my ears that the midrange took a backseat after burn-in.  i'm not certain whether burn-in is real or placebo, but either way, the mids on the m50 afer a few hundred hours use are now somewhat recessed, possibly even slightly veiled.  not too objectionable, but noticeable.

Beyerdynamic's headphones, in order of popularity, probably goes something like this: 
T1, DT880/600 ohm, and maybe then DT990/600 ohm.  Heck, some would put DT48, T50p, or others ahead of the DT990...
 
So the DT990 is somewhat under-appreciated.  However, a recent surge of interest at Head-Fi, possibly the result of outstanding prices at several vendors, led me to purchase these.  And boy am I ever glad I did.
 
So, this is my first review of any product at Head-Fi, and what better headphones to review for me than the DT990?
Of all the cans i own or have owned, these are my most favorite.  I find myself listening to them more than any other headphones in my stable (that includes DT880, K702, HD595, M50, and others).
 
So, what do the Beyer DT990/600 sound like?  Well, they have mondo bass.  So if you want a dead neutral set of cans, look elsewhere.  And they are a little brighter than neutral as well, to my ears.  So if you're very sensitive to that, look elsewhere.  And they have slightly recessed mids, so if you hate that, look elsewhere.  i know, it almost sounds like I'm bashing them.  I'm not, i just wanted to lay out the problems that listeners may perceive with these cans.
 
And now that those are out of the way, what's good about them?  Well, they have amazing imaging.  And they throw a more realistic soundstage, in my opinion, than any other headphones I've heard, and that also includes the K702.  And, the bass, if you can take it, will leave you slack-jawed.  It goes down deep, and will give you that near-subwoofer presentation, much like a denon and ultrasone.
 
The cans mate particularly well with music that isn't already bass strong, of course.  To my ears, they seem to pair excellently with classic rock.  Especially vocal -centric rock like The Beatles, for example.  Music like this allows their weaknesses to be waxed over and their strengths to shine.  For example, the bass guitar in early Beatles albums is low in the mix (to my taste), and the DT990s brings that out to a very satisfying degree.  Also, because the vocals are mixed high in these songs, the recessed mid is barely noticeable, if at all.
 
These cans are also the best built cans i've owned (along with the DT880s), and are supremely comfortable.  I can see where someone who has larger ears may have a problem with the depth of the ear cups, and their ears may touch the cushion at the back of the cup, but I personally don't experience this problem.
 
Also, to briefly compare these to the DT880s.  I will admit the DT880 is a more balanced can overall.  But, if you really like your bass, the DT880 will not provide the quantity or quality, much less the depth of bass that the DT990 does.  However, the DT880's mids are more in line with the rest of the frequency spectrum, and the highs are slightly less aggressive (and i do mean slightly). 
 
So, I can see where most would recommend the DT880 first and foremost.  But, if you're a closet basshead, these just may be your end-all open headphones...  If you're not a closet basshead, these just might turn you into one.