I power 2 M80s and 1 VP180 with my modest Denon 890 (not rated for 4 ohms) without problem. As others have suggested, give your pioneer a try first, then, if your amp shuts down or you hear distortion, you should consider a different amp. Unless you listen to very high levels for a long period of time, my guess is that you'll be fine.
I would also concord with QS8s for the sides.
As for the THX placement you linked, I would not have the back surround placed next to each other, that would defeat the purpose of stereo imaging that can happen in the back, I would spread them to each side of the couch.
