The best center speaker(if you're going to use one at all)is identical to the mains. I have no idea how closely the Infinity matches the M60s, but the closer the better.
No, it's highly unlikely that a more powerful separate amp would provide better performance. The M60s aren't "power hogs" and in fact are slightly above average in sensitivity. Your 1015 is a very powerful receiver and it's probable that you're never going to use its full capacity. Buying something with 2-3dB more of unused headroom would simply be a waste of money.
