Pros: Spectacular bass, rich, engaging mids, great soundstage depth, and excellent detail retrieval.
Cons: A bit hot in the upper-most treble, and some upper-mid coloration
After using IEMs almost exclusively, I decided that it was about time to step into the realm of full-sized cans. I wasn't about to spend $1000, but I still wanted better than mid-fi sound. My options were pretty limited, and it came down to either the HD600 or the HE-400. In the end I decided that, despite the HD600's more refined sound, that the HE-400 had what I was looking for (tight, linear bass, excellent detail, and a 3D image). So I pulled the plug on the HE-400 (and a pair of velour pads), and never looked back.
My first impression was a good one, and they sounded exactly as I expected them to based on measurements and first-hand accounts.
I'm not at all a bass-head. I find bass to be the must unimportant and non-essential part of the frequency spectrum because it provides little musical information and serves little purpose other than adding body to the music. That said, the quality of the bass is very important to me. I cannot stand liquid, texture-less bass, and I'm not a fan of elevated mid-bass either. Thankfully, the bass on the HE-400 is as far from that as possible. The response is almost perfectly linear, it's extraordinarily tight, articulate, and well-textured. It hits hard, goes low, and does everything that bass is supposed to do without ever getting in the way or disappearing. It's as close to perfect as I can imagine a headphone being in that regard.
Mids are very important to me. Nothing will ever sound right if there's trouble in the mids, and they're absolutely essential to the reproduction of the human voice. The mid-range presentation on the HE-400 is interesting. The lower mids are very full and somewhat forward, whereas the upper mids are more recessed. They're certainly colored, but it's not a coloration I'd call "offensive." The mids here are very rich and organic without ever sounding thick or syrupy. I think that's partially due to the elevated treble and excellent extension that gives vocals great air and a "breathy" sound. On some recordings the mids can sound a bit distant, but that's not an issue that I've run into very often.
The treble is the trouble spot with these cans. The treble isn't poor by any means, but it is significantly elevated in the highest octave (10kHz+). The treble can be very hot on some recordings. Some people may like the extra air that comes with this kind of treble presentation, but it's a bit much for me. I find that it obscures detail a bit, and can dominate the signature at times. However, the treble responds very well to EQ and sounds excellent with just a few dB decrease. Extension is great and the the headphones definitely do not want for air.
I've never heard a headphone at this price that presents the audio image anywhere near as well as the HE-400s do. The soundstage is open with great height and depth with excellent layering, and width that is neither unrealistically large, nor cramped. Imaging is excellent, making it easy to pin-point audio cues and effectively sorting the various layers of the soundstage.
The HE-400 signature is interesting. It doesn't really emphasize any region, but it somehow manages to be both dark and bright. It's not prefectly neutral, but it's not highly colored. It's just neutral enough and just "fun" enough to appeal to a wide range of tastes. It also responds very well to EQ (particularly in the bass, likely due to the very low distortion and linearity), though I'd avoid messing with the mids too much, as there is a bit of distortion there that can be exacerbated with EQ. However, you can tame the treble and crank up the bass as much as you want with little to no ill effect.
I've never had any comfort issues with them, and though the design seems a bit "cheap," I've not had any issues there either. They are a bit heavy, though, so keep that in mind. The stock cable isn't the greatest; it's thick, heavy, and a bit stiff. Don't be too rough with it, because there are reports of it shorting, or the sheath splitting.
I've not heard a better headphone in the $300-$400 range (including the HD600). It does just about everything right, and very little wrong (and what little it does wrong is easily fixed). For the HD600 owners out there, they make a perfect compliment to the HE-400. I've not come across a better value in the entire world of audio.