So I blew out the both speakers simultaneously on a pair of Polk Audio R20 bookshelf speakers. I am curious what would be involved in trying to find replacements in terms of fit, sound quality, etc. I believe its a 6.5" woofer. Thanks.
Make sure only the woofers are blown out. Do the tweeters still work?
To replace the woofers, your only real option is to get replacement woofers from Polk. The cabinet's bass tuning and crossover for the R20 were designed to work with that woofer. Also, you can be certain that the woofer will fit in the cutout hole.
If you are experienced at DIY speaker design, you might get lucky and find another woofer that could replace Polk's woofer. It would have to have similar T/S parameters, similar impedance and high frequency roll-off characteristics, and identical dimensions. And then again, maybe no such substitute woofer exists. I'd contact Polk.
The other question that you should be concerned with is why did both woofers blow? It is unlikely that both of your woofers were defective. Is there something wrong with your receiver or amp that could blow out the replacements as well? Or were they blown out by someone who cranked them up way too much

?