After taking a closer look at pngwn's avatar picture, the driver looks VERY similar to the AD900 drivers. Well, that should be expected, but the main differences between the two is that the ring's (is it metal or plastic?) shroud covers the cables, and the plastic on the back of the driver seems to cover the magnet. Plus, it looks like they're using the less restrictive black felt material in the center opening. The AD900s use a white felt that is tougher and more restrictive. The magnet is open too. My pro700mk2's also had the same white felt in the center opening of the magnet, but when I cut it out, there was no change in the treble (the smaller holes is where the lower mids - bass come out, and the center hole is where the upper midrange - treble comes out). The same black felt seems to be used in the smaller holes too.
So, the main sonic difference between the AD900 and the AD900x is most likely because the drivers are actually angled this time, and the earpads are thicker. That should help with the soundstaging and bass response, respectively. AT just rose the plastic on the back side of the AD900s (outside the driver), and made it thinner in the front instead of actually angling it. Oddly enough, they don't angle the drivers on the AD1000x/2000x. They do the same thing they did with the AD900s. I wonder why?
Yea. I have no idea why they don't do true angled drivers like with the W5000, W3000ANV, etc. But I do know that the sound is very pleasant. So no real complaints. :D