Even so, if you have the M80's whynot leave them at Lagre and set your crossover to 40 in the AV and instead of setting main+LFE just set the setting to LFE.
By setting them to large, the setting LFE or LFE+main has no effect, it becomes automatically LFE+mains. So the M80s will get all the frequencies and tried to play them. Before I had a sub, I had them set to large, I was showing them off to a friend and played the first 5 min of Percy Jackson and the Olympians at 0dB on the receiver. When Poseidon first steps on land, I heard one of my woofers starting to rattle. I think I damaged it during this time as the woofers were trying to play frequencies that they can't handle. I think that when you have no sub, even the LFE is sent to the mains, at least it sure feels like it as the M80s alone were able to shake my hardwood floor pretty well.
If what you're trying to achieve is M80s playing from 40Hz up and the sub taking over playing 40Hz down, set them to small and LFE only. That's the way I have mine setup and like it like that.