Pros: Solid, Taut yet Accurate Bass / Luscious Mids / Detailed yet non-fatiguing Highs / Price!
Cons: Potential Head Clamp/Weight Issues
I initially had my eyes set on the HE-500s but after some research I decided that the sound signature of the HE-400s may suit my tastes better as I predominantly listen to EDM (Tech House, Minimal, Trance, GOA, Psy). Initial impressions out of the box were quite impressive. For those of you who have yet to try a Planar Magnetic headphone - the detail and clarity are something else compared to dynamic headphones. There is no background distortion or noise at any volume as the background remains completely black.
The soundstage is adequately wide and does a great job at projecting space between instruments and vocalists, so much so in fact that I often forget I'm wearing headphones in a small, cramped room.
It's the micro-details however that really allow me become totally immersed in my music. I'd best descibe the soundscape as being in a moderately large auditorium with perfect accoustics while vocalists and primary instruments are right there beside you - maintaining the intimacy. Live, lossless recordings are a wonder to behold (Coldplay - Live 2003 anyone?)
The bass is extremely accurate and the impact is surprisingly good for a completely open headphone. I do love the fact that the bass doesn't intrude unless called upon by the track. It allows me to enjoy my tech house/minimal tracks for their deep, thumping basslines and at the same time it can take a back seat when listening to accoustic or chillout/lounge music. This is unlike most "basshead" headphones where bass is pronounced everywhere, whether it is required or not.
I would love to try these out with a tube-amp as sometimes I do feel like the M-Stage may be too neutral for my tastes for some of my darker recordings and there could be a little more sub-bass impact; however as my equipment is limited I am unable to judge if this is down to my source/amp or the headphones.
Despite my affinity towards a U-shaped sound signature, the mids are probably the highlight for me and perhaps even converted me. I have never heard vocals produced with such clarity, emotion and musicality and wouldn't have thought it achievable at this price range. Everything from Barry White to Vocal Trance is outstanding. I wouldn't say the mids are pronounced though, they are neutral if anything allowing for very accurate and believable reproductions of the human voice.
Listening to Nora Jones (Come Away With Me) is almost a haunting experience as it genuinely feels like she is singing right beside me. Her voice is portrayed with quite an ethereal tonality and there is no unnatural resonance whatsoever on her extended notes.
Mind you this is all coming from someone who doesn't really listen to jazz/accoustic so let the impressions these headphones have left with me be an indication of how impressed I am by the HE-400s.
Now I read numerous times that the highs aren't quite neutral with the HE-400s as with their older brothers but I can honestly describe them as fast, extremely well detailed yet rolled off just enough to take the edge of some hot recordings. I think HiFiMAN have done a great job with providing the detail and presence in the highs while somehow avoiding them from becoming fatiguing.
Comfort wise, the earpads are height adjustable and they do swivel, so it should fit most headshapes. I will point out that the head-clamp is stronger than usual but I'm sure that will subside after I have worn them for a while. I have had no issues with wearing the HE-400 for extended periods whatsoever but I can imagine them being a little less comforable for someone who is sensitive to head clamp. They don't sit on your head quite as softly as the Audio Technica's winged design but they are snug do a better job of staying on your ears.
My Setup: Xonar Essence STX -> Matrix M-Stage -> HifiMAN HE-400
Overall, I'd recommend the HE-400s for just about any genre as I couldn't really fault them at anything in particular. Orthodynamics are quite revealing by design so you will need to provide them with a proper source file, I've been pretty disappointed with any files <192kbps but that's a given on a quality headphone such as this.