I would very interested being an AD700 owner interested in seeing how they compare.
They're similar but there are some differences. The AD900x is still a bass-lite headphone, but it has a bit more bass than the anemic AD700, and you can actually feel that it's there. The highs are less "smooth" and sound more like standard treble which helps you hear details better. The mids remain forward but maybe not as forward as on the AD700, which is fine since I thought it's mids could be a bit too prominent at times, although it's a small difference and hard to notice. The AD900x's soundstage is about the same or slightly smaller in size compared to the AD700, but the improvements in bass and treble more than make up for it. The comfort of the headphone is worse, particularly in the earpads, but after putting on new earpads from E-Bay they're now just as comfortable or better than the stock AD700, and replacing earpads on the AD900x is somewhat easier than on the older versions. The headband has less padding too, but I've had no issues with it so far. The AD900x is a definite upgrade in sound. This headphone sounds high-quality and not tinny like some people described, and makes purchasing some more expensive similar headphones hard to justify. I got my AD900X for only $110 new which is dirt cheap for such good sound.
I've listened to the AD1000x as well. They don't have angled drivers so they're more likely to touch your ears, and they have a more forward and sharper sound overall with the same sound signature. But they're certainly not worth $150 more, they just trade a lot of soundstage for better imaging and more intimate sound. While the sound might have been refined in other ways too, the difference is so small that I can't remember them. You'd be better off getting something like the HD650 or X2 when they're on deal instead of the AD1000x in my opinion, while the AD900x remains a great choice for it's price, and it stands right at the gate of diminishing returns. If you already have an AD700 or AD900, then it's only worth the money to upgrade if they're starting to fall apart like mine were.
In the future I hope to compare the AD900x with the AKG K712 or SHP9500 and report back.
Hope that helps.
UPDATE: I've compared the AD900x with the HD598 and easily prefer the AD900x. The AD900x has more soundstage, punchier bass (although both are bass-lite) and more forward mids which I think is preferable. The AD900x has slightly brighter treble however and is more likely to cause fatigue. The AD900x is more comfortable for me once I replace the pads, since my ears touch the driver of the HD598 and it's headband isn't as soft as I would like.