Some thoughts. I am a long-time maggie owner (tympany 1(U)) and a long-time AVA owner.
As far as power goes, you'll have enough except, as Franko notes, for the loud passages. I wouldn't try to artificially create a smaller demand by bumping up the crossover frequency on the subs. That's just a performance tradeoff, and, while it's hard to predict, it may not be a good one. Subs tend to be best at the very bottom, and when you raise the frequencies they need to handle, they tend to call attention to themselves.
Despite owning maggies for 30+ years, I don't really like them that much. And they tend to not perform well at all in small rooms. T-1s in a 10x10 were painfully cramped, and MMGs in an 11x14 room were simply dreadful.
While there is certainly a subculture of people who are big, and extremely vocal, fans of maggies - there are better planar magnetic speakers out there. VMPS and Eminent Technology come to mind. There at least you have push-pull drivers so your distortion is a lot less. And true ribbon tweeters are definitely better than "quasi" ribbon tweeters which, by the way, are not actually ribbon tweeters. The VMPS and ET are also more efficient than magnepan, so will mate better with a U70.
The "best" small-room (10x10) speaker system that I have owned was a pair of biro L/1s with a sub.
I am not convinced that dragging your amp to the dealer's to listen to the 1.7s in his room will tell you anything at all about how they will sound in your room.
