^^ This is pretty spot on. The sound stage is just tremendous on the ma900. Sound stage doesn't usually mean much to me because there seems to be something about my ear anatomy that hinders the appreciation of good sound stage. Sure I can hear the difference between closed and open. But I didn't truly appreciate sound stage until I listened to the ma900.
The best example that I can give is this. Several times when listening to the ma900 and starting the music up, I thought that I had accidentally left my receiver on and that the sound was coming from my speakers. It was actually coming from the ma900.
Here's an interesting thought: could try MA900's weird "open" earpads on another headphone (HE400 or anything else full-sized really) and see if the soundstaging is earpad-dependent. I'm guessing it largely is.