As dllewel suggested, set all speakers to small and set your crossover for all speakers to 80hz. I do believe these are THX's specs as well although not exactly sure. Also turn the crossover dial on your sub all the way up to take it out of the equation. By doing this you will let your receiver handle the crossovers.
Honestly, I can't see any reason to ever have your center, or surrounds set to large. I used to have my M80's set to large with all others set to small. I figured the M80's were good enough to get a full range of sound with the crossover setting kicking everything else to the sub. After playing with different settings and crossovers I think I finally have what I feel sounds the best for my system. Here are my settings if you want to try them.
M80 small crossover set to 60hz
VP150 small crossover set to 80hz
QS8 small crossover set to 80hz
Keep in mind, every system may be different and everyone has a different ear.
