Pros: Well-textured bass, bass extension, lower mids
Cons: sometimes sibilant, soundstage a bit closed in
My source is a 2011 Macbook Pro-->Emotiva XDA-1-->Schiit Asgard-->HE-400
*Disclaimer* I have Jerpad 2.0 modded the headphones
The HE-400 does many things well, but the star of its presentation is the bass quality.
It has very good slam for an open can, but its texturing is to die for!
Cello sounds very realistic, with a roundness imparted by its bass extension.
In terms of the mids, male vocals have a very visceral, tactile quality about them, whereas female vocals seem to be missing a little bit.
The treble is rather hit or miss. It seems to be fairly well extended, but it has a bit of tinny-nes and definitely sibilance that creep in from time to time.
I find electronic, rock, and vocal tracks to be the best for this, but then again I don't listen to many other things, so take this with a grain of salt.
To compare this with the HD800, which I have only heard twice in Bay Bloor Radio, the bass has better texture, but the mids are more "accurate" sounding on the HD800.
Of course, the soundstage/imaging on the HD800 trounces the HE-400, and the treble is far better extended on the HD800.
This is only from what I remember, and I believe the Sennheiser was hooked up to a Bryston amplifier.
All-in-all, I love the HE-400 for what it does best, just wish there was a bit more presence in the upper mids to make them a little flatter.