Set you M80's to 80hz , all other frequencies above that go to the M80's and the everything below will go to your subs. That is a generalization since a crossover is not a brick wall but a gradual roll off of frequencies. I know it seems kinda silly to buy floor standers then send the lows to the subs but that's how its done. Some recievers have a setting where you can send the lows to both the mains and the sub but I find that to be too much.Also it has been suggested that floor standers tend to play better in the 60 to 150 range than bookshelves, though I had M2's and 2 subs and it sounded damn good to my ears. I have M80s and love em, go big of go home I say.
I think you can also use the M80 full range if you use 2 channel pure direct. My denon does.
