Matt, welcome. For the most part you seem to have thought this out pretty well. The one exception is that you've placed far too much weight on a 4 ohm vs 8 ohm speaker impedance rating. The M80s have an impedance around 4 ohms for less than half of the frequency range and average about 6 ohms. The slightly greater heating effect on the receiver when playing in the 4 ohm frequency range is no problem for quality receivers, certainly including your excellent 3009.
The M60s will undoubtedly work well for you, but there's no good reason to disregard the M80s.
The main advantage of a second sub would be to help smooth out the bass response if properly placed(e.g., midpoints of the side walls or front and back wall, or diagonally opposite corners), rather than simply added loudness potential.
