Too late, but I kinda wish now that I'd gone to CES to audition the 500x and 700x. I might've even been able to bring my 900x along and do some a/b comparisons, but I guess the only option now is to bite the bullet and pull the trigger on them :p
Going off of Amazon.jp, the 500x is ~$94 and the 700x is ~$137. The reviews for the 500x (messily translated from Japanese) talk about the good soundstage, nice instrument separation, and near flat FR. Some complaints about the earpads being stiff, though, and possibly switching them out with the AD500 or AD1000 pads. The only review for the AD700x talked mostly about comfort issues with the wing system, the cans slipping around while turning one's head, and "degraded comfort". Makes me curious how the build quality might have been different if the new X series had been manufactured in Japan like the originals instead of China.