I’m with Dan in that you should consider a vertical center. I’ve tested the M2 as a center for the M22s and crossed over with a sub it matches perfectly. A single M2 is also a hell of a lot less expensive than any of the horizontal centers and really isn’t that much taller than any of them. As an added bonus the M2 and M22 are Axioms best speakers when it comes to price vs. performance. Here is a review/comparison of the older v2 which has since been improved upon.
http://www.enjoythemusic.com/magazine/equipment/0509/aperion_intimus_5b_axiom_m2v2_paradigm_v6.htm
