Misfit, during my initial read, I missed the "15x20" and only saw the "3000 sq ft". Obviously you meant 300.
I have the M80's in a 16x18 ft room, and honestly, I wouldn't want to have them in a room any smaller than this. Placement has been tricky, and because of room constraints, the "sweet spot" is very narrow.
Having said this though, I love the 80's, and could never see myself "moving down" to the 60's.
If layout is not restricted by the "WAF", by all means spring for the 80's. If it is, Snippy is right - the 60's are a better choice.
Epic 80/600 + M3's + Custom Finish Algonquin V3's