Which A/V receiver for Pioneer monitor Center ?

Agree with Bob, you should try it without the center. You will not miss any information. When the A/V receiver is setup with no center, that information will be split into the L & R channels and a center image will be created, just like 2 channel. Off axis listening may be a problem depending on the distance of the L & R speakers to the screen, however if you are sitting far enough away, that might not be a problem. If you find the voices are not to your liking, then add the center, and you will have a level adjustment for the center speaker. Nevertheless, you have nothing to lose in trying no center, and should it work to your liking, you don't have to find a place for it.

Tls49- Thanks very much. It started out as fun, but honestly the center channel was more than I was looking to get in to. I really just wanted to improve what I already have, and I feel the Plasma built-in speaker is the weak point. I like the idea of tip toeing in to this w/ just 2-channel....

If sound quality is important, Yamaha receivers also get high marks. The RX-V377 isn't as stylish as the Marantz but is $150 cheaper and would probably do just fine. I also agree to start without center speaker if you're not sitting far off axis. Just another option, and best of luck.