You may be able to replace the drivers in your speakers by contacting
Polk directly -- they may sell you replacement drivers.
There are probably several online companies I'd expect to offer these drivers as well.
However, never let it be said I'm not willing to sell someone elses money!
I really think that the offerings from
SVS offer the best bang for the buck these days. Of course, everything audio is personal, but I don't think you could go wrong with their speakers.
JCD