I have a Harman Kardon AVR-125 (40 watts per channel) 5.1 DD/DTS AV receiver. My mains are Axiom M22TIses, my rears are Mzero's (I got them demo for less than 50%).

What center would match the M22s? I want something that's "even" with them, doesn't dwarf them or be too small.

The VP-100 seems like it would match the M22 best, since it has the same drivers. However, I and veryone here is going to say the same thing:

If you can afford it, definitely get the VP-150. It is noticeably better than the VP-100 and is still a great match for the M22s.