Good thoughts Bridge and Mojo. Forgot that it may help to also do what is known as the Subwoofer crawl with yuor M80s. If you can place an M80 in or as close to your listening spot as possible. Then crawl around while playing something with more continuous bass notes when you find where you have even and distinguishable bass notes. Put speakers there.
A good track to use is Jennifer Warnes Ballad of the Runaway Horse.
There's a placement technique called "Master Set" Where in you can set your mains to your rooms modes\nulls etc. All versions of such placement recommend This song specifically for setup.
The Master set placement involves moving your speakers mere millimeters and a couple hours of time. To which is a whole lot easier if your speakers are already in a decent place out of nulls etc.
P.S. it sounds like a lot of stuff but once you get into it. It can become quite entertaining, with huge sound stage differences within a couple of centimeters.
