Where is your center channel located? What amplification is driving it (and what is driving the M60s)? Do you frequently have the volume turned up pretty high? Is your surround system calibrated?
Dual center channels can be one solution, but there may be a way to get your existing one working as you like. I could be wrong, but I'd guess from a perfectionist viewpoint, trying to synch two center channel speakers (assuming different locations) can be a chore, and could potentially muddy the waters if not done right (this is not something I have experience with, so this assumption may be incorrect... I'm going on what I know of acoustics, time alignment of speakers, and the like).
