Like I said, it's a cosmetic change. There's more bass because of the thicker earpads. They help because of the better seal around your ears, which equates to less roll off on the lower frequencies and greater sound pressure. The AD900 and the AD900x are angled differently. The AD900 raises the plastic on the rear of the enclosure while making the front thinner, thus making the drivers take position of that slope when you put the headphones on your head. The AD900x's drivers are angled without any slope on the headphone enclosure, which is the reason why the drivers are less prone to touching your ears.
In spite of the reviewer dis'ing us head-fi members lol, he does make a comparison of the ad900x versus the ad900 (also comparing them against the hd650 and srh840):
I can't really agree or not with his comparisons, because I haven't tried those other headphones. But I couldn't agree more with his positive take on the ad900x's.
Edited by waynes world - 2/15/13 at 3:17pm
If you read the AD900 vs AD900x first, and the other ones later, you'll see that the change isn't that dramatic. AD900x= more comfortable, bassier AD900s. Once I transplant my AD2000 drivers (coming in within next few days) in my modded AD900s, It's going to be extremely hard for me to buy any other headphones for a while. To think I bought the drivers for $94...hifi products sure are expensive. 
