The Polk center is a sateliite speaker, so it has its limitations. Moving up to something like the Pioneer C22 would be a big upgrade, despite it not matching your Sony towers. I would say get the Pioneer center, and then if you go and get the towers during black friday, you will have a matching front sound stage. Your surrounds don't need to match, so the Polks will do a fine job as your rears and surrounds.
Samsung PN60F5300 | Denon AVR-1713 | EMP E55Ti | EMP E56Ci | EMP E5Bi | Outlaw Ultra-X12