Four M80's would be hard to utilize effectively in a 20x20 room. As others have said, they want you to be a good 12 (preferably more) feet away. They also typically want to be away from the walls, so 4 of them would be non-ideal in a 20x20 room; placement would likely be constrained to not-so-good positions. I think the decision between the M80 and M60 for mains comes down to how loud you need/want 2-channel material. I have M80 mains in a ~ 16x24 room with a high cathedral ceiling, and if it weren't for entertaining and my habit for loud 2-channel sources, M60's would work just dandy. The M80's are certainly overkill for movies; with 5 channels and subwoofer driven, the room gets very loud with the gain set fairly low on the amplifier and the M80's giving no indication of overdriving. No one with their hearing still intact could sit in my family room with the M80's maxed on 5.1 material, it'd be painful.
The QS8's are nice surrounds. I also agree with spiffnme; multichannel audio generally uses the surrounds for things like crowd noise from a live concert recording, etc. There are exceptions, but I find a lot of the exceptions annoying, not unlike listening to early stereo recordings where there was plenty of playing with the technology w/o regard to the long-term listening experience (I know Hendrix wasn't sprinting back and forth on the soundstage :-)). I just don't think we're ever going to find multichannel audio appealing if the surrounds go way beyond being mostly ambient sounds. It's not how we experience live performances (if you're on stage, you're not sorrounded 360 degrees by individual instuments.. I think you only are if you're in the center of the orchestra pit, and most of us have never had tha experience and even fewer would say they prefer it to say the conductor's position).
So I'd pick your mains for 2-channel considerations, and your surrounds for movies. Today, the requirements for DVD-Audio and SACD aren't much different than that for movies, for the reasons above. In fact, I dare say I have been happier with some of my DVD concert video than most SACD and DVD-Audio titles. The reason being that it takes a good recording engineer to get multichannel right, if the objective is to recreate the live experience versus parading the technology. For example, I think the James Taylor 'Live At the Beacon Theater' DVD (recorded in 1998) holds up well to most DVD-Audio and SACD titles, as does the more recent Diana Krall 'Live In Paris' (2001).
