I think I would bump up the sub to an EP500 other than that you have chosen a great system.
I think the QS8s will work great back there, just not quite as enveloping as when they are mounted to either side.
You could always pull extra wire down from those rear positions to the floor and run it around/under the base boards to the side position.
I have my QS8's next to the couch (I mean *RIGHT* next) to the rear couch and it works fine. I have them on the FMS stand that Axiom sells. Since QS8's have a speaker on the bottom, you can't use any old stand.
Also, if cost is a factor, I would bump M80's down to 60's and go for the EP500. I have the Epic-80/500 setup and I *love* it!!
