The simple fact of the matter is that you are the one in control of the volume so ie you are in control of whether the room gets overpowered or not. Both the M60s and M80s COULD overpower the room with enough volume but at reasonable levels both would be fine. For the record, my room is almost the exact size as the one you've indicated, in fact a touch smaller and I'm using M80s with a VP150...at -5 on my Denon(loud!) it starts to overpower my room(ears too). Furniture, rugs and other sound absorbing thingees (or lack of) in your room have a great effect on the acoustics as well. For instance, if you have bare walls, hardwood floors(no rug) and little furniture, you'll easily overpower the room even at moderately higher volume.
