I had the opportunity to hear HE-500 under the EU loaner program organized by the EU importer (thank you SievekingSound). I am not really an audiophile (I don’t usually hear critically or do I notice small details in soundstage, etc) but I do enjoy good sound. I have, also, very limited sound experience, both with live music and high end equipment, so please keep this in mind when reading the below and use a fair amount of grain of salt.
 
I have compared HE-500 sound with Sennheiser HD-650 (‘new’ version) which is the only HQ headphone I own. I used mostly European jazz, folk and classical music for evaluating and comparing the two.
DAC/AMP used are CEntrance DACmini  and Qables iQube V2. I used a laptop win7 computer as a source with Lilith audio player.
 
1. Comfort
HE-500 is heavier than HD-650 and clamps more. Its pads are more stiff. Its cable is also more stiff and more heavy (personally, I believe that this cable is a joke and should be replaced with a lighter and more flexible one). All these contribute to a significant less comfortable headphone. Its sound, also, unequalized (see below, paragraph 2.2), has much more volume on highs. Although these highs are not harsh or sibilant, they tend to tire my hearing system more easily, but this easily changed with the help of proper equalisation.
There is something else, also, that was fatiguing me with HE-500, but I couldn’t pinpoint it. Maybe it is a kind of resonance, something in the sound which I couldn’t really define causing me a slight fatigue (even after equalising the sound - see 2.2 below). Fortunately this fatigue was very slight, and I believe that, if it is truly some kind of resonance, I could pinpoint it and eliminate it with proper equalisation if I had enough time.
 
2. Sonic impressions
 
2.1 Smooth, clear sound
The first thing that stroke me upon hearing HE-500 is its clear, free sound. I cannot really describe it, it is like you are hearing less distortion (but, of course, HD-650 has already low distortion). I do not really know why so big difference in favour of HE-500, probably the different drive technology, but the fact remains: Notes from HE-500 are sooo much clearer, distortion -free, grain free. In comparison HD-650 seems ‘electric’ (I mean it seemed to have a kind of distortion usually associated with electric instruments), especially in the voices. Did I say HE-500 has a more clear, free sound?  
 
2.2. More extended at the ends, more low & high frequencies
From the first few minutes it was quite clear that HE-500 have much more bass (although a bit less defined) and treble than HD-650. This was very flattering for the sound of HE-500.
By curiosity, I have compare the frequency response measurements of both headphones, using the data found on Tyll Hertsens InnerFidelity  measurements page. I combined the two frequency responses and it turned out very similar:
 

 
 
With yellow and light blue-green is HD-650, with red and dark blue is HE-500. As you can see the FR graphs are very similar indeed, with a bit more bass (about 3dB at  30Hz) and more treble (about 7dB at 9KHz). HD-650 has almost 4dB more output at 2KHz and 3 dB around 200Hz.
This flatters HE-500 sound much. So, based on Tyll’s data I equalize accordingly HE-500 to ameliorate these differences. I do not claim that I completely and successfully make the frequency response of HE-500 exactly the same, that cannot be done by FR graph alone as this paper from Harman Acoustics notes. Resonances in the audio spectrum, even light ones (with low Q and dB) are clearly audible, although they do not really show in frequency response graphs. But the equalisation I used based on Tyll’s measurements helped ameliorate the big flattering of more prominent and extended frequency limits of HE-500, so I was able to compare it more easily with the HD-650, without the need of extended listening sessions.
Unequalized, HE-500 clearly wins HD-650 but it is also a bit more tiresome, because of the more prominent highs.
 
 
2.3 Midrange
Ah, the midrange is truly remarkable on HE-500. Very much more engaging and ‘creamy’ than HD-650. Both female and male vocals are more palpable, more intimate, lifelike. Even with less than stellar recordings, the midrange magic of HE-500 makes voices very enjoyable.
One thing that I noticed, though, even when I equalized the 4dB difference with HD-650 at 2KHz, is that HE-500 has more low frequency volume in vocals (I believe some resonance about 800-900Hz, I haven’t truly pinpoint it). This makes vocals more ‘creamy’, but I feel it masks some details.
 
 
2.4 Bass
Bass, even after equalised to match the FR of HD-650 has more impact and energy on HE-500, but it is also more slow and not so much controlled. There are more upper bass frequencies (probably a 2 dB resonance around 100Hz with Q around 1, I haven’t exactly pinpoint it). This makes bass ‘thick', but I think maybe contributes to the slowness of bass notes.
 
2.5 Treble
Treble is more prominent and extended, even with the use of equalisation based on Tyll’s data. But they are quite sweet and clear. There is no more sibilance issues than with HD-650.
Some may feel that the unequalized treble is way too much more than HD-650. In the majority of recordings I don’t believe to be so, but on some ‘bright’ recordings, the unequalized treble of HE-500 is indeed too much.
 
2.6 Soundstage
One advantage of HE-500 is the more centre - focused soundstage. Maybe its soundstage is less wide than HD-650, but it is more seamless and more centre focused (HD-640 seems to have three distinct soundstage areas, left, middle and right with less organs in between).
 
2.7 Placement tolerance
With HD-650, even slight changes of the headphone placement on one’s head, effects the frequency response (especially in the highs) and, accordingly, soundstage. This is not the case with HE-500. I found the sound pretty much the same when placed in a logical position on my head.
 
2.8 Sound with less than stellar recordings
That was a concern of mine. I do not want to separate myself from the numerous technically troublesome but excellent artistically recordings I enjoy with HD-650. I found, with much relief, that although the technical flows were a bit more apparent with HE-500, it didn’t distract me from enjoyment because the added midrange creaminess and overall sound clarity counter balanced it.
 
2.9 Amplifiers
I used mainly DACmini for evaluating both headphones. I found DACmini quite capable of driving HE-500, but at least two other members of HeadFi found that a stronger amplifier will drive HE-500 better. Unfortunately, a stronger amplifier was not available to test this.
 
2.9.1 With iQube as source
Just for the sake of it, I used also the portable iQube. Although it’s a portable amplifier with a mere 80mW at 32 Ohms per channel it did drive HE-500 quite well, but with limited volume. Classical music and some jazz / folk with high dynamic range drive iQube to its clipping with HE-500 more easily than HD-650.
Even with iQube, HE-500 was better overall, with clearer sound. It has more coherent and holographic stereo image but a little smaller. The lows were much more extended and maybe a tad more defined. Big difference was on voices, HE-500 presented voices much more intimate, with fluidity and smoothness. Voices on HD-650 seemed ‘electric’ in comparison. Highs were more prominent but quite soft.
 
3. Conclusion
 
Comparing HE-500 with HD-650 which I am familiar with, I found it better overall, with clearer and less distorted sound, ‘creamy’ midrange and more extended and prominent bass and highs, which flatter its sound significantly. Of course, there is a significant price difference between the two, but my personal view is that it is worth it.
 
 
Edit:
Hifiman upgraded the cable and the pads and reduced the price significantly. With the new price, I believe, this phone is, given adequate amplification (Hifiman recomments 1W/ch), a ‘giant killer’.
The cable is now silver. It is thin and light and very comfortable, but I cannot comment on any sound changes (I have only an HE-6 type intermediate cable).
The included pads are new and two pairs. One pair is made of pleather (protein made artificial leather), which is softer and at first more comfortable. But it is also more shallow and you may find, like me, that the outer parts of your ear is touching the inside metal grill (covered by a thin sheet of felt). After a while this gets pretty annoying. These pads make the sound signature somewhat bassier.
The velour pads are updated also, but I cannot comment on these because I have the old ones.