Hello everyone!
I just bought my HE-400 off of a member on here last week and received them yesterday. All I can say is wow. I was excited for these as they're my first costly purchase ($300) and I have never hear planar drivers before. My other cans are the ATH-M50x, HD598, HFI-580, ATH-IM50 and DX 120.
**I am new to the audio world, so please forgive my terminology**
The first thing I noticed is that the bass feels like it has impact, unlike my other headphones. I really don't know how to explain it. The clarity is unlike any other headphone I have tried, although the sibilance can kill it. Thankfully these seem to EQ nicely and you can get that well under control. I have the stock pads on but have read that the velour pads make a big difference in the sound. Considering I am more than satisfied with the sound, I don't think I'll be going for the velour pads just yet. I've also noticed that everyone is saying these cans are heavy and uncomfortable, but I don't feel that way. Sure, they are definitely heavier than other headphones, but they rest so nicely on my head that I can't even tell I'm wearing them at some points.
Bottom line - GET A PAIR!
tl;dr - Good sound, bass impact, sibilant but can EQ, somewhat heavy.