My take on these cans:
I've had the HE-400s for about year, and am now running them through a V-Dac II into a Burson soloist.
Argon 5057 Cables between the amp and dac. My pair have the revision two casing (White-ish).
For modifications, I swapped pleathers for velours and removed the mesh that was glued onto the grills, no problems caused from that so far so thumbs up. It would probably be a safer bet to keep the hifimans under cover once in a while so they don't collect too much dust. These two mods are a must for anyone wanting to get the most out of these cans, however I wouldn't recommend the mesh mod if you have an already bright system as it slightly adds more treble and contributes to more fatigue (at least that's what I have observed).
Genres I listen to: Metal of all kind, female vocals/acoustic, pop, dubstep & electronic. I can only comment on my own system because i've never owned anything else. The sound you get from these is very full, bass is perfect, prominent in the mix but not too much that it takes away from anything else, mids are really nice, gives guitar and vocals that extra richness. I have had to EQ the highs a tad, because these can get harsh with some less stelar recording mixes, Soundstage and imaging is good, you can pinpoint all the different elements in the song. One of the only negative things that I can find in the sound (and I may be over-emphasizing this) would be the really wide soundstage (more on the sides and less in the centre) that may make music that's played on a stage seem less realistic. This doesn't really bother me but if you're really looking for that speaker-like soundstage, other cans may do this better. This wide soundstage works perfect with more digital music like electronic etc.
The HE-400s build quality (driver casing/headband,etc) seems really durable, but these cans really need some refining. There are some things I feel could have been better, the 'Hifiman He-400's logo paint has started to fade, the R/L channels have completely dissapeared over and the cable has started to ware right before the connectors. Also, comfort is one of the big cons of this otherwise, really great sounding headphone, so take this into consideration (I am looking into upgrading to the revised models, HE-560).
What else can I say..I wouldn't say this headphone responds alot with better equipment, probably because of its efficiency. The V-Dac and Burson amp did make subtle changes but if you're budget is limited, going with less expensive options won't set you far away from the headphone's full potential. Only thing I would look into a headphone from this would be more detail, less brightness, more realistic imaging, and refinement in build and comfort. I think synergy is pretty good, the Burson Soloist gives the HE-400's all the power it needs (does not need this much), and the Musical Fidelity Dac adds warmth so anyone looking at this setup, go for it. Conclusion: All rounder cans, great value and a nice step into the audiophile world. Feel free to message me about the headphones, dac or amp. and thanks for reading my review.
EDIT: Hifiman has released two new models, the HE-400i and the HE-560, these are discontinued.