Jack, as my reply to Tony in his earlier "pros and cons" thread indicates, although "comb filtering" exists, in my view it should be dismissed as being a significant concern. As Ian said about two years ago in a reply and speaker designer Dick Pierce has also stated, in a live listening room reflections create comb filtering all over the place and any small additional amount from two separated speakers playing the same sound isn't significant.
In my own multi-channel(pre-Axiom)setup, I use a vertical center speaker just slightly bigger than an M2, which is the same as the mains. One is adequate in my experience and is the logical choice for a center speaker, as in my view the horizontally aligned center is a solution in search of a (significant)problem. Two should be fine, and the center image won't move out any farther than the speaker on that side as the listener moves to one side or the other.
