I've been doing a lot of research on budget subwoofers recently. And I've consistently read the Dayton Audio Subs, such as the 10 inch Sub-1000 outperform the Polk PSW10 in every thread/review I've come across. They're on sale too! But I'm currently waiting on the 12 inch sub to go on sale (Sub-1200) before I purchase.
http://www.parts-express.com/dayton-audio-sub-1000-10-100-watt-powered-subwoofer--300-628