Yeah... I hate to say it but with an 18x30 room I can't fault you for thinking about M80s. That is the kind of room they were made for.

Then again, if your system and listening position are all in one part of the room (ie distance from speakers to listener is ~10 feet or so) then the M60s will be fine. Either way, you're in BigHonkinSub country

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8