Oddly enough, about a week ago, I dusted off my old Alpha 7R and rigged it to my RS6's and compared it to my A65+ and I found the Alpha timed slightly better and was a tad richer in the bass, although not quite as open as the A65+. So now I'm giving half-a-thought about maybe adding a power amp to it.
Back to your original question, I agree with d_a_n, by the way, avoid the A90. The A70 when I tried it lacked presence as well as punch compared to the A65+. So no, it is not an upgrade [IMHO]. At best it's a step sideways.
Bi-amping a power amp seems the most logical upgrade.