folgers,

The M60ti sounds pretty much exactly like the M80ti. The M80s can just play louder. The main reason to go with the M80 over the M60 is if you have a large room you need to fill with sound. Additionaly, the M80s need more "breathing room" than the M60s and sound best if you're sitting further away ( > 10 ft.), which can be a problem in a smaller room.

If you do get the M80s, you'll need to set the receiver to the 4 Ohm mode. Since the QS8s and the VP150 you already have are 6 Ohm speakers, and 6 is less than 8 and greater than 4, I'd set the receiver to the 4 Ohm setting.