Personally, i find that the HE-400i is a bit of a chameleon. When I first got them after selling my ZMF Vibro (which is very, very lush and on the polite side), I thought the HE-400i sounded clean, hard-hitting, fast and aggressive.
Fast-forward a few months to where I get my TH-600, and the HE-400i sounds lush and polite in comparison, almost like how I remember the Vibro.
Overall though, I think the 400i has lush (but not extra lush--it's too fast) mids that are slightly forward. Bass extension is excellent and impact is average to above average, quantity is neutral. If you are used to drier sounding bass I could see the bass sounding a little soft, but never loose or sloppy IMO. Air is average to below average for open headphones, but you'd have to be used to the best closed headphones in the world to think these sound closed. IMO they are obviously more open sounding than the TH600, which itself is exceptionally open for a closed headphone.