Conclusion: I purchased this subwoofer to use in a custom enclosure in the footwell of my Miata. The airspace for the box is about .25cf. I am powering it with a PPI p900.4 on the rear channels bridged. I estimate somewhere between 300-350w rms is going to this sub. It sounds great!
Conclusion: Really great product as long as you know what you want. Polk is known for its crisp and sharp punches. If you're just looking to be absurdly loud and rattle all the metal in your car, go buy the Sony X or Kicker Comp 12's. I have 2 of these 8s in my car and they sound awesome.
Conclusion: Not enough bass for my Jeep JK Wrangler using the factory enclosure and an Alpine PDX-5. Lasted about 3 months and completely blown listening to mainly Phish and Jack Johnson, etc. Not even rap. Went with an RE Audio SL-10 and I am much happier!
Conclusion: I bought this woofer and the 8" angled box for my GMC S-15 PU. This is a great value and fits behind the seat. While listening, I am still at times surprised at how good this woofer sounds.