The MW8103 driver those use is really hard to find and it's an odd size so you really can't just go to Parts Express or Radio Shack and get a replacement.
They do come up on Ebay every so often - in fact right now there is a seller with 3 drivers left for $50 each... and that's actually a good price for these since they're so hard to find.
You just have to ask yourself if it's worth $50 to fix an older Polk speaker...
Have you talked to Polk? This may seem like a long shot, but you never know.
Is there any visible damage to the speaker? Can you post good quality (well lit, low noise) pictures of the drivers?
Assuming you have narrowed the issue down to a single driver, have you taken the offending driver out of its cabinet? Any visible damage there as well?
If the driver's cone and basket look good, the problem may be something as simple as a bad surround. Unfortunately, surround repair kits for off-size drivers are difficult to find as well, but you may be able to make something work.
At the end of the day, however, these speakers are not going to be worth putting hundreds of dollars into, in my opinion, unless they have value to you above and beyond their original MSRP.