upgrading Polk woofer to Def tech woofer?

No, you can not do that. No offense but this question that you asked shows you severe lack of understanding about how this stuff works. You can't just unscrew your Polk woofer and put a new woofer on and expect to improve the sound quality. Every speaker has a crossover that is designed for that particular enclosure and combination of drivers. If you change out one of the drivers then you also have to change the crossover, which is very very complicated. If you want better sound quality, then sell your current setup and buy a different one. (note- I am absolutely hate Polk speakers as I feel like they are overpriced junk).

That's the least of the potential issues. Foremost is that the replacement driver must have T/S specs compatible with the cabinet design, then it must have sensitivity and frequency response that will work with the tweeter.