BobG
My guess is that the M80's or M60's have all the bass below somewhere near 300hz going to the bass drivers.
A note on what sirquack said, that some bass below 80hz will go to the main speakers because the roll of is gradual... Although this may be true, I have heard that most digital crossovers to subs are very high order crossovers, which means very little bass at all below 80hz (or the selected crossover point) will actually go to the main speakers. This information may be available from the manufacturer of your receiver, but I have never seen it in a manual. You may have to ask if you really want to know, but I think it's a safe bet that it's a very high order crossover, hence, very little bass below the crossover point will go to the main speakers.
