First of all, and most important, the search function for these forums is abysmal.
Secondly, Hockey Life is a really huge store with tons of selection. I quite like spending time there glazing around.
Third, quite simply, all the Axioms speakers will match so closely that you wouldn't notice anything abnormal with their centre channels or main speakers (regardless of so much other hype people type about comb filtering, etc. etc.).
However, that being said, there is a pretty distinct difference between the M50s and M60s.
The extra (dedicated) midrange driver really does add a ton more detail IMO. The M50s are a bit smaller (and hence less expensive) and what some people refer to as "more laid back". The sound is a bit more recessed, a bit less detailed.
A few years ago a few friends got together to buy the M40s (small brother to the M50s now discontinued) and i had a chance to pair them against the M60s.
You can read my thoughts in this post.
http://www.axiomaudio.com/boards/ubbthreads.php/topics/98061/The_M40s_vs_M60s
