The major thing you compromise when moving from an open design to a closed design is the soundstage, so just know you're not going to recreate that airy feel in a closed can. Coming from the AD900x, are you positive you're looking for depth and not detail (both of which I think that can does pretty well, big fan of the AD lineup)?
Well I know one sacrifice soundstage when going to a closed can, but I'm rather sensitive to noise in the surounding so I'm trying to minimize them as much as possible. I want to be able to use the headphone in what I prefer to call "Footstep-simulators" (think counter-strike), where small details and accuracy of the sound are important. Whether that would be depth or detail I'm looking for the most, I'm not sure. Apart from that I'm pretty certain that as long as it's a nice headphone and sounds well, I can live with pretty much any sound signature .