Let me echo the sentiments above. I'd progress: QS8, Center, Sub Rears.
Your M80s are going to give you great bass throughout the experience. Assuming your amp can drive that low bass sound, you will gain, maybe, a little more "oomph" when the bass hits you in the chest, but not much. It certainly won't blow you away.
Same with a phantom center in the meantime. Although VERY important for movie soundtracks, the sound from a phantom center channel will still appear to be coming from the correct place with the twin M80s.
Not so with surround speakers. It's very Very VERY hard for digital sound processors to fool your ear into believing a sound is coming from the sides or behind you by using delay, phase and volume cues. Better to remove the algorythmic mumbo-jumbo and go with the real thing: surrounds. This will actually blow your mind. I think it's the single most impressive aspect of home theater that really draws you into the experience. Without good surrounds, you're really just making the TV speaker experience louder and maybe crisper. This is the best way to get more expansive sound.
