Absolutely not an issue. Far side of the room from the speakers has a catwalk up above between the two upstairs bedrooms. That's about as far as you can get away from them and and they sound just as good up there as they do down closer at floor level. I'm sure a trained ear could speak to the differences but people like to hang out up there during parties and often comment on the excellent sound.

My floors are laminate, Lots of windows but they are mostly on the wall behind the speakers. The far wall has a stairs and lots of angles. I'm no expert on sound diffusion but I think any diffusion effects from your room makeup will effect an M60 or an M80 pretty equally.

I'm not saying an M80 doesn't have a subtle edge in some respects. Again, I'm just trying to remove worries that an M60 will not perform up to snuff if they suit your budget better as they did me. I wanted two pairs of mains so I went with the 60s and although I know in my head that the M80s sound just a bit better, I never heard them so I can't imagine better sound than I have now.

With great power comes Awesome irresponsibility.