Got my He-400's about 5 days ago. Liking them, for the most part.
Comfort-wise, they don't hold a candle to my Senn HD590's (or DT770's, or HD448's), but that was to be expected from their weight. I feel I'm sensitive to uncomfortable headphones (had to return my Grados, hate all on-ears, etc.), so I'm actually impressed by their comfort considering my expectations were low. Oddly, I find the pleathers to be more comfortable than the velours. Perhaps the velours can be massaged to soften them, but I also noticed more sibilance with them, so I went back to the pleathers. I'm definitely going to give them another shot though, since I didn't pick a track to AB. The biggest comfort issue to me now is the headband. Bending it to fit the curvature of my head helped, but I think I'm going to have to buy some extra padding.
Sound-wise, the first thing I noticed compared to my HD590's was the treble presence. Considering how much time I've spent with the 590's (which I adore, btw), it's a bit jarring. I'm finding myself unable to listen to poor-quality recordings/masters which was never a problem with the 590's. Obviously, I'm not faulting the HE-400's for revealing what was there all along, it just never occurred to me that it'd be such an issue with so many of my albums. As someone who listens to the DT770's everyday at work, I was a little afraid I'd find the bass lacking. Not in the slightest, I actually much prefer it to the DT770. However, I do find the mid-bass lacking, making the low-end guitar crunch in 80's-00's indie rock less impactful and fun than the HD590's. The HE-400 is aces though with electronic music and jazz. The Knife & Mingus on repeat. Not quite sure how I feel about it with classical yet.
The soundstage is similar to the 590--which is to say, I'm quite pleased with it. The imaging seems superior, perhaps aided by treble-emphasis (that is, relative to my status quo).
For anyone interested in an Apogee Duet (Firewire) + HE-400 combo, I have zero complaints and no desire to upgrade the DAC/amp. FYI "really loud" is about 80% on the Duet for me.
Thanks to the community here for turning me on to the HE-400...never would've heard of them otherwise! The law of diminishing returns is in full effect, but I'm quite happy so far and looking forward to spending more time with these chubby cans. It was between these and the HD650's. Because I had no plans of getting rid of my HD590's, I figured I'd get more variety switching to HiFiMan. I suppose I'll never truly know until I try the HD650's.