Not a lot of action in this thread!
I just received a pair of AD900Xs today. I bought them as a second pair to complement my incoming ZMF Vibros. A little premature as I don't actually have the Vibros yet, but they are closed so I figured a open pair with a big soundstage would work nice.
So far, I'm digging these. I probably couldn't live with them as my only headphones, but they will do well for acoustic, ambient, and other genres where a big soundstage is valuable.
Detail retrieval is better than I was expecting for a headphone in this price range. The last bit of black space and low-level detail isn't quite there compared to my HE-400s, but its not a big problem.
The soundstage isn't as wide as I was expecting, honestly. I thought these were supposed to be even bigger than the Q701. Not in terms of width, but they do have more depth and more of an overall sense of openness and air. I can see what people mean when they say the imaging is a bit fuzzy. These headphones don't do layering very well, but the more nebulous imaging imparts a sense of space in its own way. Again, I wouldn't be happy if these were my only headphones but as a second pair I can appreciate them for what they are.
Timbre is very nice with the exception of bass guitar (not enough body IMO). Strings, voices, etc. sound great.
These are clean and bright without being peaky or harsh, so far anyway. A tiny bit of sibilance is present,
Bass extension is adequate, but the deep bass falls way back in the mix. On the other hand, these do have a great sense of snap and impact on drums.
I'll add a full review at some point once I've had these for a while.