You could go with the Polk center or Pioneer C22 with the condition that you upgrade the fronts at some point. Sony may be coming out with an updated version of those speakers, so maybe wait a little while and see.
Or, if you have the vertical space, use one of the bookshelves as a center. Traditional horizontal center speakers aren't the best because the horizontal drivers can cause lobing. A more idea configuration is a vertically oriented set of drivers, like a bookshelf/tower.
Denon AVR-1713 | EMP E55Ti | EMP E56Ci | EMP E5Bi | BIC F-12