The general recommendation is to crossover your mains to your sub at 80Hz, but personally I would look to cross them over a little lower considering your M80s are very capable.

In my particular room, I get smoother bass by crossing over my M60s at 40Hz and using the Denon version of "Plus" as well. I also have an outboard 200 WPC amp to take much of the burden off of my receiver.

No offense, but it's not likely that you are actually bi-amped.

::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::