One thing I wasn't expecting was for the soundstage to expand in all directions, especially becoming much taller. Now it feels like the music encompasses my entire head instead of just being spread out around me in a 2D plane like it does on the AD900X. I'm guessing this is due to the "newly designed diffuser" and "newly developed baffle with fin-shaped airflow path" which AT says controls air dispersion for a more spacious sound. When I read that on the box, I thought it was just marketing speak, but WOW, it's a huge difference.
The air diffuser should definitely have some kind of impact. It's the same principle as having curved blades in a fan to improve air flow or maybe spread the turbulence of the air movement over a wider range of frequencies. I have no idea just HOW much of an effect it would have though given that the diffuser is relatively thin in both models.
If you're having discomfort with the drivers touching your ears with the AD2k/x, I really recommend putting Denon pads on them. My pair has them and they're really comfy. The thicker back portion of the pads really helps to distance the drivers from your ears and also makes it easier to tell which cup is left or right :P
Doesn't the increased spacing decrease the bass response?