Hi 2packer,
To expand on what JohnK and others have posted, by setting the M80s to an 80-Hz crossover or lower, you are not "wasting" the bass output capabilities of the M80s.
All crossovers have a "slope" (think ski slope), which gradually lessens the amount of bass to the M80s' woofers so they don't have to "work" as hard (i.e. the cone excursion--the distance it moves back and forth--is reduced); when cone excursion is reduced, distortion is also reduced, and your subwoofer will take over the bulk of low-frequency generation powered by its own amplifier. This frees up more amplifier power from the Denon's power supply for the M80s and your other speakers to handle peaks (bass frequencies use up a big chunk of amplifier power).
Note also that the M80s woofers will continue to reproduce upper bass up to around 200 Hz, which will give lots of support to those octaves of music.
Regards,
Alan
