Hi bdpf,
LFE+Main settings allows both the main speakers, the M80's in this case to be set to large playing low frequency content that otherwise would be sent to the subwoofer. Now if you have the crossover of the sub set at 80hz this is the area below the subwoofer will be utilized as well. The best I can describe it as a "double bass" feature.
This is a bad idea in general for a few reasons even for floorstanders.
1) Blu-ray movies have a lot of content below 40hz with even some subsonic content with extreme dynamic range. Loudspeakers are not meant to reproduce such frequencies at reference levels even the M80's. The M80's will sound a lot cleaner and handle more power set to small crossed over to at least 40hz.
2) You have these powerful amps and by having the M80's set to large you are robbing them unecessarily of power and headroom for the mains because instead of having the subwoofer handling the bass scenes which a well designed subwoofer should do well on most occasions (that has its own amp) the speakers are drawing power as well and this is the area the current draw will be the most. The amp for the speakers will be drawing a ton of current and working harder along with the speakers because of the low impedance of the M80's in the low to lower frequencies during those scenes.
Essentially by doing this you are making your amp work harder as well as your speakers especially at loud listening levels. Set to small, use a higher crossover point, a sub and the speakers will handle more power and sound cleaner especially during dynamic passages. Your risk of damaging any of your equipment is much less as well.
