My room is 12x20x9, so almost exactly the same, but it is a sealed room. I have not heard the M60s in my room, I've only heard them elsewhere in another much more challenging room (described above) and they performed quite admirably.
Without having done an A/B test, I would just be repeating what others have said about the comparisons, but I'll do it anyway now that I've told you that's all I'm doing....
Many folks here have commented that the M80s play fuller and deeper at low volumes as well and of course they have the ability to go even louder at times when you need it.
In my case, I was sitting on a dedicated HT budget that was originally designed for more expensive speakers, so going with Axiom's best lineup didn't stretch what I'd set aside (in fact it was a bargain). If I was approaching with a tighter budget and was trying to decide between the 60s and 80s, I _might_ buy the 60s and spend the extra money saved on a better sub, but that's just me.
I'm a big fan of bass, and having heard how amazing the 60s sound, I would probably choose to put the extra money there if pressed. That's just me though! Hopefully that ramble helps.
