Depending upon the capabilities of your processor, you may wish to try the M60s at something lower than 80Hz. I have mine set at 40Hz, as I have bass issues in my room and find that I like to have the M60s contributing some bass as well to give my three sources instead of the single sub.
It's a little more taxing on my amp, and the Dolby recommendation is 80Hz, so it's not magic bullet... but might not hurt to try.
Also, my receiver has the capability to send the bass signal to BOTH the sub and the main speakers, but other receivers allow only one or the other. If yours is like this, I would just go ahead and move it to 80Hz.
Don't forget to set the "crossover" on your sub as high as it can go (assuming your sub has a crossover adjustment), since you're already telling the receiver to handle that switchover from "Mains" to "Sub".
