I would go with M3s rather than M2s, just to get the best timbre matching between mains and surrounds. They cost about the same and M3s are only a couple of inches bigger.

QS4s would also be nice -- they will give a completely different effect, less "wow" but more "gee, this sounds like a big movie theater".

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8