We are getting somewhat into semantics here. The 5 1/4" "woofer" in the M2 is the same as the midrange driver in the M60s as opposed to the 6 1/2" woofers. While the 5 1/4" drivers are called woofers when they are the largest driver in the speaker I was just trying to clarify they are the same as the midrange in the M60 and that the M3 would have the same tweeter and woofer as the M60. Since deep base response is not what we are looking for in a center channel I don't believe this is really an issue if you were trying to match M60's. Where this might be more of a consideration is if you were using the M40 or M50 for mains since they both lack the 5 1/4" midrange driver.
I have M50's with a VP150 center. I don't find the voicing between the VP150"s 5 1/4" "woofers" and the M50's 6 1/2" woofers to be a problem but others have commented on a slight difference in voicing.
Given what the center channel mainly does, which is produce dialog it makes sence that the 5 1/4" midrange driver as a "woofer" makes the most sense.
I'm not sure about this, but I think the older M2ti have a 3/4" tweeter rather than the 1" tweeter which now runs across the Axiom line with the exception of the QS4. It used the same 5 1/4" midrange/woofer as they do now. It gets confusing because the driver mix has changed towards more uniformity in the last year or so. I believe that the QS4 is the only speaker now that does not use any of the three drivers common to all other Axiom speakers (1", 5 1/4", and 6 1/2").
For multi-channel music with M40's and 50's perhaps the M3's would be a better choice of centers. I rarely listen to music in a surround mode that uses the center channel. I find stereo recordings generally send too much information to the center when played in surround modes. The exceptions to this are music recorded in DTS multi-channel and some recordings made in large resonant halls. Like a recoding I have that combines pipe organ and pan pipes recorded in a large stone church.
Bottom line, I think your approach has a lot of merit.
