Okay, I've had a few days to play around with the Ad900x now and I can give you my opinion on them.
Comfort: You will need to do the rubberband "modding" to make them comfortable. They slide down all the time if you don't do this since the clamping force isn't enough to properly hold them in position. It's really not a big deal, but something you may want to consider. The earcups are extremely soft and comfortable, they don't get too hot either. They are pretty big though, I had the feeling that they were pressing against my jaw at the beginning, but I got used to that in a day or two and now they are really comfortable for me. They are also pretty lightweight, which is a plus if you wear them for a long time. Overall I'm really happy with the level of comfort those headphones provide.
Build quality: Everything feels solid. They have that high quality feel to them, they aren't flimsy or appear like they could fall apart every second (which actually was a concern for me judging from the pictures alone). The cable is nice and thick and both the connector and the 1/4 inch extension are gold plated and, again, just feel solid.
Sound quality (music): I used the headphones with my onboard sound, no soundcard or a DAC or anything fancy. I tested all kinds of genres like metal (Amon Amarth, Cripper, Blind Guardian, Behemoth, Slayer, The Apocalyptic Riders, Ensiferum), Jazz (the Seatbelts), Swing/Electroswing (Circus contraption, Little Violet, Swing Republic, Tape Five), House (Daft Punk), Classical music (Beethoven, Mozart, Brahms) and even a bit of J-Pop. They definitely don't sound flat or neutral, there is a clear emphasis on the mids and highs. The bass is definitely there and it is quite punchy and accurate, but if you are a bass head you won't be happy with the AD900x. So, everything that depends on the highs (female vocalists, guitar solos, violins, etc.) benefits from the extreme clarity. They worked really well for metal and rock (I also didn't have the feeling that growling or vocalists with deep voices were at a noticeable disadvantage here) and they were outstanding for classical music and Jazz/Swing. The only real "problem" I had was when I started listening to daft punk. Stuff like Robot Rock depends on the bass too much to sound perfectly "right" with the ad900x. The bass was definitely there and, as I metioned earlier, quite accurate, but the airy feel they have to them simply wasn't perfect for this kind of music. I imagine that the same logic applies to techno, drum n' bass and other bass heavy genres. If you like the sound signature they provide then they sound amazing, especially an orchestra or a bigband really benefit from the large soundstage and the emphatic highs/mids. I quite like them.
Sound Quality (gaming): Positional accuracy was amazing. Even without any additional software I was able to pinpoint enemy gunfire and footsteps very accurately in FPS games like BF, Arma or CS. Again, the focus on the highs and mids really helps you out here. At the same time explosions still packed a puch without drowning out any other noises. They seemed like the perfect balance between immersion and positional accuracy to me. I also tried using Razer Surround to further improve the soundwhoring capability. It worked pretty well and definitely gave them a nice boost, but even without any virtual surround sound they work surprisingly well.
Conclusion: I'd say that they are a really good all-rounder for what they are. If you know what you want (a VERY open set of headphones) and you like their sound signature then go for it. I don't know if they are worth the upgrade form the AD700x, I can just tell you that they suit my needs and that they do everything I use them for extremely well. Also, keep in mind that you don't want to use them if you listen to badly encoded music on youtube or other low quality sources (that also includes fake, bad quality radio calls in video games) since the emphasis on the highs really hurts your ears if you have cracking sounds or weird hissing in your music/game.