polk audio powered subwoofer PSW110 repar parts

I doubt you'll finds "parts" for $200 system. If you know it's the pot - open the speaker, take the pot out (you will probably have to desolder it first), it should have markings indicating the resistance. Then go to RadioShack and look for something with same mounting pattern.

As for parts - $100 will get you "plug-and-play solution.