Dollar for dollar, the Polks are great, you may want to look around as sometimes the Polks can be found for $80-90. I would also recommend these 12's if you want to spend just a bit more, the quality is a bit higher and the base is just incredible.
http://www.amazon.com/BIC-America-F12-475-Watt-Subwoofer/dp/B0015A8Y5M/ref=sr_1_1