George, I have a very similar problem, but in my case rears and matching (I'm using that term loosely) my Shahinian main 2CH speakers. I know the "right" answer in my case is Shahinians for the back (NOT cheap) and an amp. I'm not that much into MC, and even a reasonable tonal match would be good. And in my case if they can be self powered all the better.
I'm not worried about a center. My speakers are "polyradial" to begin with (think omnidirectional), so a phantom center though my MC controller would probably be good enough.