60 or 660 all the way! I know psychologically your mind tricks you into believing bigger has got to be better but it's simply not an absolute. You are more likely to get better imaging, soundstage and bass from the M60 compared to the M80. And the 60 will likely disappear better.
The 660 will provide a wider and deeper soundstage with improved imaging and dynamics at louder volumes. And it might also disappear better than the 60. I say might because my M50s disappear just great and I expect the 60 to do the same.
My buddy loves the M100s he stole from me. He fell in love with them in the first 10 seconds of listening. The character of the sound is more important to him than acoustic disappearance. In typical rooms, even though the M100s are bigger and more expensive, they suffer more from "position error" than M50, M5, M3, and M2 and their bass is not as linear. So although the M100s sound more like instruments compared to the M5, I like the M5 more because of their imaging, soundstage and disappearing act.
I went with active omnis so I can have it all. And now I have even better fidelity, imaging, soundstage and disappearing than M100 or M5.
Cork, I was very happy with M5s, and I would have been happy with M2s in my 4200 cu ft space, but I went down a deep, dark chasm!
