I had a very brief listen to a 2000x today. In the past I've owned or heard the 700/900/2000 (non-x). Maybe I have an odd-shaped skull, but they actually fit me better than the non-x models and I find it very comfortable.

They didn't feel particularly fast to me. Not bad by any means, but it didn't jump out like a stat. The bass is good for an open headphone. It definitely rolls off, but gently. It doesn't have that "airy" texture that the old non-x cans have; not sure if that's a good or bad thing. There was some grain in the treble, possibly distortion, but I didn't notice any overly sharp peaks (or at least none that bothered me).

I think I'd still take an HD600 over this, although the 2000x are much more comfortable for me.