After about 60 hours of pink-noise burn-in I haven't heard any changes in the sound. The first 20 hours produced a dramatic improvement in SQ (I'm usually hesitant to use such a strong descriptor but it applies here) after which the changes were quite subtle. Overall, I feel they are a pretty big step up from the HD650s, but not in every single category.
The treble is more natural, extended and forgiving. Yes that's right, more forgiving than the HD650. I had sibilance before burn-in and now it's completely gone, even on most of my badly recorded tracks that are a bit harsh and congested on the HD650+DHC Blackout Cable. This was probably the biggest surprise in my listening tests considering all that's been said about the tipped up treble of the HE-5 around here. I may just have gotten a smooth set, but they have the most realistic and unfatiguing treble I've heard. Quantitatively, there's definitely more of it than the HD650, but it's not sibilant or harsh.
The bass has better attack, is more textured, airy and resonant (in a positive sense) with a little less overall weight than the HD650s (most noticeably in the midbass). I do feel the HD650s are slightly better damped though. This was a very subtle finding. I have to listen very closely to hear this and have to investigate further whether it's the recording or if the HE-5's are extending the decay of the bass a bit, because the effect isn't reproducible in all tracks.
Edit: Upon further listening, I believe the HE-5 aren't extending the decay, or if they are, then it is very slight and troublesome only in certain recordings. I believe the main difference is the HD650's more prominent mid-bass masking the natural decay and adding a bit of punch of their own. However, I prefer this added punch over the HE-5's presentation. I feel the bass quality on the HE-5 is great overall and is quite textured and detailed. I would like a bit more punch, but that's just my preference. I should note that I listen at low volumes (usually around 80dB or less) and I feel this isn't an issue at higher volumes where the mid-bass seems to bloom a bit more.
If I may digress a bit from the bass, one thing I've found is that the HE-5's, unlike any other headphone I've heard, keep sounding better and more open and engaging as the volume is turned up. The soundstage keeps getting larger and larger, the sound more enveloping...this is in direct contrast to every other headphone I've heard which become fatiguing at higher volumes regardless of their sound signature.
Moving onto resolution, I can hear more low-level details on the HE-5 and a better sense of space because of their remarkable ability to render ambient cues in recordings. In many of my well-recorded tracks, the soundstage is tremendous, reaching far beyond the actual earcups without sounding distant or weak...definitely had me some "wow" moments coming from the HD650 here. Imaging is also a step up from the HD6x0 and is accurate and conveyed with startling resolution at times.
Dynamics are also noticeably better on the HE-5 with the same source/amp, and I think this is partly because of the HE-5's more forward nature and better attack/transients in the treble.
Now, the biggest strength of these cans has to be the midrange, as good as the other areas are. It's just life-like, instrument timbres are very natural and the aural image of them in space is huge (a lot of this will be recording dependent though). Often times, I genuinely felt the singer or instrumentalist was in the room with me, an effect that hasn't occurred very often with my other headphones, atleast not to this extent. I don't think much more needs to be said about the midrange other than the fact that it just sounds real. Compared to the HD650, the mids are recessed though, but once again not unnaturally so. I would say the HD650s aren't far behind here, although they emphasize the lower midrange more while the HE-5 show off the upper midrange more. They're both quite engaging in their own way, but for overall realism, the nod goes to the HE-5.
As for comfort, they are quite good, as the headband can be bent to match the shape of the listener's head. For me, the HE-5s are atleast as good as the other cans I've previously owned, and I've had some comfortable cans in the past (re Beyers/Senns etc). Comfort will ofcourse be subjective, and I guess I'm lucky in this regard that I can almost forget they're on my head. The only sensory stimulus I have of them being on my head comes from their weight (which BTW is very reasonable IMO).
Earlier in this thread, Skylab purported that complex passages seem to sound congested on the HE-5 and since a lot of my listening is done with complex orchestral pieces, I was concerned with his finding. Well, I'm glad to report that I tried hard to find any signs of congestion with the most demanding tracks I have (of all different genres BTW) and I never once felt the HE-5s were strained or confusing passages. In fact, I was a bit confused (delighted?) to find the opposite; one of the things that impressed me most was their ability to delineate complex parts and produce such a natural, visceral feeling of a live venue.
Now for a few negative points. I find these are more helpful sometimes than positive ramblings, so here they are.
1) Decent build quality, but the wood is fragile and is prone to cracking. Edit: Fang says as of January, this issue has been resolved, but time will tell for certain. 2) The bass doesn't extend as far as the HD650 (they're very close though), and 20hz is inaudible unless the volume is turned way up. The good thing though is the tone is clean and undistorted. Anyways, the HE-5s start to roll off near the 35hz mark, and at 25hz SPL decreases sharply. 3) They could use more punch in the bass (I think this is caused by a fairly flat frequency response from the bass to the lower midrange as I measured, some may prefer this actually). There is also the lack of damping to consider, and the resultant overhang of the decay I mentioned earlier. Ofcourse, I found this to be a very subtle effect, but it is noticeable at times, particularly with genres that rely heavily on quick pronounced bass beats such as progressive house. 4) Absolutely no isolation. This should go without saying but I think they leak as much sound out as they do in. 5) They take a bit more volume than I'm used to to sound their best.
Of the "cons" I feel the first 3 are most significant and should be considered before buying these headphones, especially point # 1 (One). The last 2 are really just nitpicking.
I haven't heard the HD800s or T1's so my impressions may not be worth much to those who want comparisons to headphones in the same weight class as the HE-5s. However, for anyone wondering, the HE-5s best the HD6x0, DT990, Pro750 and all the lesser cans I own or have tried.
Equipment Used: Foobar WASAPI > MisterX y2 DAC > Rockhopper M³ Amp + σ11 Power Supply > HiFiMAN HE-5 Orthodynamic
Edit: I found some frequency graphs that I believe go well with my findings: Google Translate