Tom, i'm not sure why you don't use the receiver crossover (or the sub crossover) to find the appropriate blend of the sub and mains. Does your receiver not have a crossover setting?
If not, the subwoofers all do in the form of a knob on the back.
I have my M60s set to large in my receiver so they play full range, and i've crossed over my subwoofer with its x-over knob to about the 50-55Hz level which is where i find it blends best with the M60s for my bass preference.
As the M60s roll off naturally, the sub picks up around this frequency. If it sounds too full, bloated or muddy, i can turn down the sub x-over or its gain/volume until i find a good mix.
