bigjohn, if you set your receiver's crossover at 60hz, then all speakers set to "small" will receive no signal below that point. Instead, all those bass signals will be sent to your sub. Setting the M60s to "large" means they will get a full signal, including the LFE. This requires your receiver to amplify more of a signal and your sub less.
I set my M60s to large and toyed with the phase switch until it sounded right. Turn your subs crossover knob all the way up. You will probably then need to adjust the sub volume between music and movies to get both to sound right.
