Your M80s can certainly handle lots of bass. No fear there. However, if you have a quality subwoofer, it is normally better to let it handle the LFE and let the mains concentrate on 80 Hz plus(or whatever your crossover is set to). No need to reproduce the bass twice. In fact, they call this "double bass" and it is not normally recommended although a few people actually like it. The crossover is not a hard cut-off so they will still do some bass below that number, it is more of a slope.
Also, what Mark was wondering about being bi-amped is, you are undoubtedly using 2 separate amp channels per speaker and you probably have removed the clips tethering the binding posts together but have you also gone to the trouble of using completely power separate amps & removing the crossovers in the speakers to be replaced with active crossovers before they even get to the binding posts.
If not, then what you have done is 'passive' bi-amping. It's fun to do but each driver is still receiving essentially the same full range signal going to both binding posts and there is no effect to sound quality (except at a subliminal level... which is where the "no offense" part comes in.)
If you have done the complete 'active' bi-amping job, please let us know the specifics and what you thought of the results. Not many of us have gone this route so it would be fun to get your experience.
With great power comes Awesome irresponsibility.