Well, hopefully you don't get bombarded too hard with anti-Bose posts.
I would say that you should at least try to find a center channel speaker that timbre matches well with your L/R front speakers. I can't say that the Polk doesn't because I haven't heard these speakers together, but I'm guessing that they don't. If you like the way the Bose speakers sound, then look for a center channel in the Bose line that is similar. If you prefer the sound of the Polks, then you may want to replace the Bose with Polk L/R front speakers.
If you haven't already, use a sound pressure level meter and make sure that all your speakers are calibrated to the same level. Remember also that your room has a very large impact on the sound of your system. Try moving the speakers around a bit and see if that improves the sound first. It could be as simple as that to make the system sound better.