I bought an M2 for a center channel and I found it to be lacking too much in the lower end for male vocals. It seems odd that you found it to work good in that application. Right now I'm pairing the M2 with the VP100 and I still find it a little missing in the male vocals, so I'll be swapping out the VP100 for the VP150 sometime soon to see if that helps. I don't have the room for the VP180 or I'd go that route.
i really think it depends on the size of the room, distance from the speakers and whether or not you've got a good sub cut in just right.
i have a VP180 in my main HT room which is much, much bigger than the room my M2 center is being used in and they both sound great for the space that they are in. the fact is that in my smaller secondary HT setup in my basement office, i use 3 M2v3's for my fronts and i was absolutely shocked at how good they sound as fronts paired with a good sub to bring up the low end. for me, that wouldn't work out very well with my main HT room as i believe my M80v3's would over power the M2v3 being used as a center.
both can be used as a center, no question, like i said it depends on the size of the room and distance your sitting from the speakers and of course the listeners preference which may change everything!
