I had the pdr-8 for a number of years before i bought my svs. I found the paradigm to be very boomy.
I don't think any of the subs you mention are sheilded. I know my svs isn't, my old 27 inch tv the corner towards the sub turned green in about 3 days...
Anyways when i was listening to subs last summer i did listen to the polks and liked them very much, i liked the forward firing design and it looks cool with the grill off. I'd say they had an overall better sound than the paradigm subs. I guess it depends on what kind of sound you are looking for. In my opinion the polks had greater accuracy and cleaner sound where the paradigms are more geared towards spl.
Have you looked into the new svs pb10-isd or any of the hsu subs? Both are great companies worthy or your attention if you are interested in purchasing a new sub. Just my 2 cents.