A bit of background:
 
Now to my review:
I think my demo of the HifiMan RE-400 was one of the fastest purchasing decisions I have made.  Compared to the other in ears I was testing, they just sounded so "right" with a flat midrange and such natural timbres!  I have had other similarly quick demos but those mostly involve me ripping headphones off my head because of how badly coloured they are, but that is another story.  RE-400's were clearly the most faithful earphones I had heard that day regardless of price (I had listened some earphones costing 20x the price of RE-400!)
 
Anyway I think it is customary to break reviews down into sections, maybe BASS, MIDS, TREBLE etc. so I might do the same:
 
BASS:  Flat.  Not elevated or rolled.  Just flat down to 20 Hz like a planar.  Tight.  Not boomy.  Detailed.  Acoustic.  Fast.  A little shy when not driven with enough power.  Excellent.
 
MIDS:  No peaks or dips.  Clear.  Perfect timbre.  Fast and clean.  Resolving - this is not the same kind of (overly) airy sense of resolution you get with BA earphones, and subjectively there is not as much finer microdetail as some of the multi BA solutions, but the overall faithfulness of timbre is higher and resolution is hardly lacking*.  *Actually I think part of the subjective impression of resolution might be down to the frequency balance which I will discuss that later.
 
TREBLE:  Good timbre, moderately good extension.  I can still make out all the cymbals and acoustic instruments all sound remarkably lifelike.  There is some lack of air due to rolloff in the high treble.  Mid treble also needs some boosting with EQ.  I added +6dB centered at 6kHz and +10dB centered at 16kHz, however you may find different adjustments more to your taste.
 
I use these earphones with my Sony ZX2 DAP, which from experience may be slightly lacking the power to make the bass wake up but, interestingly the RE-400 are not holding back this high end DAP. The RE-400 will improve with better gear such as Chord's Mojo, or even some other higher powered DAPs.  The RE-400s are incredibly transparent and revealing.  Colourations in source components or amplifiers are laid bare.  They are not harsh (though there is some very slight sibilance splash).  Yes it makes sense to use this (USD) $100 earphone with high end portable gear like Chord Mojo, Sony ZX2, Astell & Kern AK240 etc.
 
I mostly listen to classical and jazz, however I still listen to some heavy metal.  With classical and jazz the technical prowess of these earphones is hard to beat at any price.  For heavy metal, the technical ability and truthfulness is usually also good, however poorly recorded material will sound however it sounds.  I can just back off the treble EQ to compensate if the recording is too harsh for high volume listening.  When playing acoustic music you want to be able to appreciate the full performance in all it's nuance and dynamics.  Timbres are very convincing.
 
Anyway I could continue to waffle about the technical ability of this earphone, however at the end of the day it will be up to the listener and his gear, music and tastes to determine if they like this earphone or not.  I almost overlooked these earphones, firstly because of the price (I think many others do as well), but secondly because I was not aware of just how good they are regardless of lack of hype.  I am sure that the few listeners with good ears who have discovered these gems are happy that their performance is not widely hyped.  It's a small miracle that this level of sound quality can be found for so little money, especially considering the general trend in the market towards more costly designs.  I recommend for everyone to try these earphones, especially audiophiles who value neutral, transparent, and low distortion sound.  Do NOT make the mistake to overlook these earphones because of the low price.

In the months leading up to the release of the EF-6, I was considering the few options available for amplifying the HE-6, there is basically speaker amplifyers and a few pre-amplifiers designed to power the AKG K-1000, which was prior to the HE-6 the most notoriously difficult headphone to drive on the market.  A few things stood out about the EF-6, there was the handsome good looks, the speaker amplifier style case with large heatsinks on either side designed to cool the discrete, class-A biased output stages, all the way to 5 WPC into 50 Ohms.  Reading over the spec sheet it was hard to fault the design, small things like the solid looking shields over what I can only guess are the massive transformer and filter caps.  Everything just looked incredibly well built and engineered.  There are few amplifiers on the market claiming class A operation that actually operate with class A switching for the full power output, EF-6 is not one of these - it operates in class A for the full rated power output.
 
Upon receiving the HE-6 the first thing I noticed was the weight - it almost felt moderately light when packed in the boxes it ships in.  One box reveals another, and that reveals one heavy beast of an amplifier which appears to outweigh it's size.  I can say that it looks every bit as good as Iron_Dreamer's beautiful photographs show it to be.  Once hooked up to my DAC, the EF-6 takes a pause to switch on.  All the connectors and controls feel solid and affirmative in their operation, the gold plated Canare RCA's look great, and the XLR and TRS connectors at the front are recessed into the fascia.  For reference this is a single ended amplifier, the 4 pin XLR at the front though allows compatibility with the original HE-6 cable as well as any other headphones terminated for balanced operation, which means no XLR to TRS adapter is needed.
 
One of the first things I noticed about this amplifier is how hot it runs - it is definitely in class-A but definitely not hot enough to be uncomfortable to touch.  After about 100 hours of use I felt confident that the amplifier had indeed fully burned in.  The way I would best describe the EF-6 sound quality would be powerful and yet refined.  The shy and ill defined bass I had previously come to expect from the HE-6 when used with a lesser amplifier was replaced by deep and powerful yet controlled bass which I never knew the HE-6 was capable of producing.  This newfound bass ability of the HE-6 was also coupled with excellent dynamics, percussive impacts are rendered in a more life-like way.  The bass and percussion are now able to drive the pace of music.  The treble, I found extended and refined without any fatigue, and yet not at all lacking in clarity or definition.  Vocals from the EF-6 are rendered with excellent transparency completely free from gloss or glare, just as it should be.
Overall I found the amplifier clear, transparent and yet able to avoid the fatigue typically expected from the HE-6.  In fact I would call the pairing one of the more neutral in frequency balance I have owned.  The EF-6 is incredibly powerful and possesses a massive amount of dynamic headroom.  When I received the unit it was set to -10dB gain on the back, and with good reason.  With a source putting out 2.5V (typical line level) I never listen past 11 O-clock on the volume control.  I can't imagine anyone needing to use 0 Gain setting with the HE-6 unless their source puts out a very low voltage level.
 
All in all I thought the EF-6 the perfect compliment to the HE-6 headphone, and I would imagine it would work equally well with many other power hungry headphones.  The amplifier is very transparent and refined, and powerful, qualities which IMO should bode well with any headphone.  There is no risk of blowing up your headphones - Hifiman have that covered with a gradual power on and power off.  With the EF-6 you can be assured that your are providing the HE-6 with enough power to hear it as it should sound, and that there will be no problems typically associated with with synergy or lack of power.  I would see little reason to use a speaker amplifier when the EF-6 can extract such performance from the HE-6.

Thanks to WNBC for his review, I am in complete agreement with most of the points he makes, however I will put my experience in my own words.  Trevor, who runs Norse is completely professional to deal with, as well as clearly dedicated to creating some of the most natural and transparent and beautifully crafted cables that can be found at any price.  He is generous with his time and willing to tailor his creations in any way possible to your aesthetic preferences.  I cannot possibly forsee an experience as a customer which would surpass commissioning a cable from Norse Audio.
 
The cable is beautifully crafted, the finish on the wood splitter is excellent, and the cable looks and feels fantastic to use.  Norse Audio has sourced carribean rosewood to match that on the LCD-2, which it definitely does.  This cable looks like it belongs with the headphone.
 
What you can expect from the sonics is some of the most natural, balanced and transparent sound possible from the LCD-2.  Once you hear this cable, you will start to notice how lifelike and just plain right acoustic instruments sound.  In direct comparison most other cables I have tried start to sound artificial and coloured by comparison.
 
This cable has an incredible ability to sound both natural and engaging at the same time, while also maintaining a very good headstage.  Other cables may provide a sense of larger soundstage, but this usually is also accompanied by a distancing from the musicians.  With this cable the presence of every musician is palpable and natural, full of nuance and natural tone&timber.  You get a real sense that the sound waves are travelling directly from the musicians and reaching you without passing through any recording equipment, more than with any other cable I have tried.  Much of this can be attributed to the aforementioned First Person perspective this cable provides - you ARE the conductor, you are IN the recording studio.  
 
This cable is as detailed as any other I have tried, but not in a high definition/etched manner as can often be the case.  The bass is full of texture and dynamism, yet still very balanced, moreso any of the stock cables I have tried.  Bass is tight and impactful and full of definition, yet not vague and overbearing as it can be with the stock cable.  it does not bleed into the mids.  The midramge is palpable and natural, vocal transients are clearly and naturally reproduced, woodwinds, brass and strings all benefit from the natural upper midrange presentation and sound more realistic than ever.  I would not classify this as brightness - it is more a lack of coloration of this frequency which other cables seem to exhibit, save for the Nucleotide wire cable I have been testing.
 
I highly recommend this cable if the characteristics I described appeal to you.
 

Please excuse my inexperience in reviewing audio gear - however I wish to contribute what I can to information on this product.
 
First a bit of background - I previously came from sony ex700, earsonics sm3, and HifiMan reZERO.  es5 is my first custom, and the closest earphone I have heard to it is the earsonics SM3 which I will be using as the basis for comparison in many areas.
 
Starting with bass, the quality is quite good, impact is moderate and speed keeps up quite well also, but it does not seem as tight as bass on reZERO, but definitely more present.  Others have described a midbass boost and I would have no trouble believing this.  Extension seems adequate.
 
Midrange is the strong suite - it is smooth and slightly forward, as well as slightly warm and rounded in its note as described elsewhere.  Female vocals are truly lush and sound amazing.  Male voices are quite good, but the same lushness tends to soften them a little.  The coherency of voices is excellent, but overall i prefer a little more edge with tenor voices to enhance the sense of vividness.  However with some recordings the forward mids can get a little uncouth as per earsonics sm3.
 
Treble is detailed, and given my sources and hearing I cant comment on extension.  What I can comment is that the timbre can seem a little dry at times.  There isn't the same degree of "wow that cymbal sounds like its actually made of brass" that I got from the sm3 (its close though).  Treble detail though is very good, as is speed.  But I am being really picky here and many people don't seem to have a problem here.
 
The es5 has been described as smooth or rounded in its presentation of timbres, and this I find accurate.  The edge and agression on some other earphones such as reZERO is toned down a little.  These are not fatiguing earphones.  Overall impact, pace and presence is excellent which lends to really great PRAT.  Instrument separation is excellent, it is really too easy to follow individual instruments or hand on a piano.
 
I find these very comfortable from first fit, which is really something for a custom.  Isolation is excellent.  As with any custom though, chewing feels uncomfortable and weird.  Overall these are a very well rounded high grade custom with great comfort, and a very coherent sound reproduction that hides it's crossovers well.  What I consider shortcomings in the sound reproduction may to others be strengths, or non-issues at all.

These are quite a nice sounding headphone for compressed music or bad sources. 
 
They are very smooth, balanced, if slightly grainy in sound at first, but later smooth out.
 
Compared to high end earphones they sound a little muddy, but if the mp3 is low quality this wont matter.  These don't have much bass or treble extension, official frequency response claims are nothing like what you hear - and a completely misleading form of data.
 
These are decent don't get me wrong, but nowhere near the top earphones from Westone, Shure etc.  
 
Not really audiophile material, but inoffensive and more than up to the task of portable audio.

This review will likely be all but redundant soon, as Sony seems to be constantly re-branding its earphones.  But I believe these are still available, and relatively good value.
 
They have excellent clarity, but not much soundstage.  Separation is quite adequate.
 
These headphones have great and natural bass impact, a smooth midrange, and a horrible sibilant lower treble.  The upper treble is actually quite good, if not great for the price.
 
Basically once you EQ off the sibilance, there is little treble left.  Sony engineers must really be deaf...  These are quite literally unbearable without an equaliser, unless you have some high frequency hearing loss.  This really limits their application and usability.

These were my first hi-fi close headphones, moving from Sony mdr-v700DJ (yuck).  I use these mostly for heavy metal for reference.
 
Firstly, bass is tight and punchy, at least as fast as Grados, but not as fast as k701 (is anything?) but doesn't have quite enough impact in the lower bass.
 
Isolation is great, no sound leaks out, and not much in.  Comfort similarly is good, they are light and don't threaten to fall off if you lean over - thats a good thing right?
 
They are not what I would call dark, not compared to ah-ad2000 or Shure 840, have a rather up-front presentation, but instrument separation is good.  Sound is coherent and fairly musical, and relatively warm

Yes there is a tendency to sound nasal when under-amplified, so there may be better mobile choices.

They can sound a bit harsh at times, maybe due to the upfront presentation, maybe due to the abovementioned 'mid-high fuzziness,' but this really isn't a deal breaker.

For rock, and heavy metal they do very well.  For pop music there are probably better headphones for vocals.  Classical music tends to sound a bit flat and unmusical, and soundstage isn't the best, so not a great choice for classical.

If I payed $200 for them I would be quite happy...

 

This really is a personal, subjective review, but hopefully you can learn something from it.
 
I really shouldn't have bought these headphones, as it is well publicised that these are analytical, and possibly dry; however the problem is to understand how you will subjectively respond to something dry and analytical (or what to expect musically.)
 
What do these headphones do for music?  They bring out subtle details in the texture of the sound (as opposed to the tonality) as well as highlighting the timing/rhythmic variations with great detail.  
 
Now this may sound good, but what does it do subjectively?  At times it makes music sound "awkward" by exposing small timing flaws which human musicians make. 
 
At the same time the tonality of the music is dealt with in a rather critical/objective fashion, no romance at all, making things sound dry at times compared with other headphones.
 
Thirdly, the detail of these headphones reveals some lovely aspects of the musicianship, how the musicians hands move over the strings etc.  
 
However, it also reveals flaws in the recording/production, such as musicians moving around on their seats, changing pages, people in the audience coughing etc.
 
Ask yourself are these the aspects of your music you want to hear, along with tonal indifference?  I will let you answer that question.
 
I found these headphones to be suited more to classical music, but then people coughing or moving around gets annoying.  well produced rock music also occasionally sounds good.
 
Lastly these are definitely a headphone that is best to find second hand, as their sound is very divisive, a love/hate affair
 
NOTE: I didn't try these with a tube amp - I used a Burson HA 160