My suggestion, and one that "I" like because it's just exactly what I have-is to go with a pair of M50s for mains, and a pair of M3s for the other set!(But yeah, the M3 is, unfortunately, a bit larger than the M2)

They sound nearly identical to each other and the M3s are a great Stand-Alone pair. In a small space they work very well without a sub.(I swapped out my other speakers in the dining room system with my surround M3s the other day to see exactly how they work, and they are a wonderfull speaker in that application!) I even had another post just last night stating that my future plans are for M3s in the dining room AND, later, in the kitchen! Really!!

M3s and M50s are not for everybody, but for those predisposed toward that type of sound, they work just great!

Hey!! Something to think about!!
Rich.