Drew, this seems to be getting tangled-up with a discussion of amplification, rather than trying another center channel option. If you did try two center speakers there's no general problem with doing so and no different amplifier would be necessarily required; an impedance problem shouldn't be assumed.
The simplest thing to try first, to see if there's a problem with the VP150 itself, is to just set no center on the 2600, so that your mains handle the center in "phantom" mode. Not much more complicated is to put one M2 at the center.
