My impression so far is that they are slightly better, but only slightly. The AD700s are so incredibly good, however, that even that slight improvement is a seriously impressive accomplishment.
Since I had the money at hand and didn't have to make any sacrifices to get them, I find it a worthwhile upgrade. If I had had to scrimp and save to do it, however, it would not have been worth it (at least so far.)
One area where there is a definite, nonsubtle improvement is build quality.
If the AD700s are a diamond in the rough, then the AD900s are that same diamond cut and polished.
These will keep the upgrade demon at bay for a looong time (at least until I can afford HD800s.)