Hi Autoboy,
Everyone here has offered some good advice, however, even in your very large space, if the M80s are not out of phase, and your listening area is not in a big fat null because of room cancellation (which would almost eliminate audible bass), then you should still hear some good solid decent bass from the M80s.
I think there may be a wiring issue or you may have clipped the Onkyo on bass-heavy material and damaged the woofers. Leave the system off and gently press in each woofer back and forth to see if there is any scraping sound. If there is, the woofer is gone and will need replacement.
Overall, in a room that size, you could benefit from a separate power amp for the front channels and at least one powerful subwoofer, an EP500 to start. I've always found the THX guarantee dubious in terms of power output levels on rated receivers.
Check the wiring of the M80s first to ensure they're in phase, and then each M80 woofer (with the system turned off) for damage.
Good luck. Beautiful room, by the way.
Regards,
Alan
