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.