Bill,
I would not set the crossover for the center and the surrounds much higher than 80Hz. The main reason is that the higher the frequency components your sub has to play, the more the sub sound become "localizable" -- the illusion of the deep bass sounds coming from 5 main channels instead of the sub will be more likely to be destroyed. Regardless of the published frequency specs for the VP150 and QS8, these speakers are designed so that they comfortably perform in a 5.1 setting with 80Hz crossover to the sub.
The optimal crossover frequency for the M80s depends heavily on the room acoustics and your own sonic preferences. Only the actual experiments with different crossover frequencies will tell what's best for you in the particular room. I would experiment with 40, 60 and 80Hz, without making any preconceived "theoretical" assumptions that one of these should be best. Rest assured that even if you end up with 80Hz crossover, you will not be "wasting" anything out of the M80s. Relieving them of the power- and driver-excursion-hungry bass duties may well result in an improved sound quality in the mid-bass region and above.
