<font color='#000000'>By "work with", I'm guessing you mean voice matching the new center speaker with your older Polk speakers. My understanding is that the ear/brain processes sound a bit differently between sound arriving from the front and rear, so voice matching your rear speakers to the fronts isn't as critical- close is good enough in most cases.
From the age of your speakers, I'm guessing you're upgrading from 2 channels- so if you're going to do the full 7.1 thing, you'll be on the hook for 5 new speakers in any case- 6 if you don't already have a subwoofer.
If memory serves, Polk follows the New England tradition in terms of neutral sound, so I think that a reasonable match Center could be had, as you seem to like the speakers, you could identify a current set that you like, buy all the rest and try them out with your current Polks, then just buy the new mains if you're not satisfied.
I guess it comes down to balancing how much you like your current speakers, how much money you have to spend on this, and exactly what types of listening/viewing material are most important to you.
My 'sagest' advice is that you're the one that needs to be happy, and if you're satisfied, don't let anyone tell you that you shouldn't be (this isn't to say you shouldn't attempt to educate yourself and train your ear). And also, as it's about the sound, don't pay for differences you can't hear.</font>
