I listened to the Polk PWS-12, Sony SA-WM500 and SA-WX700 at my local hifi stores and I like the PWS-12 better than the SA-WM500. The Polk gives more solid and tighter bass at high volume thanks to its high dynamic power amp. However, the Sony SA-WX700 is an excellent subwoofer with dual 10" drivers and 250WRMS amplifier. I think the SA-WX700 is about $210, street price including shipping with no taxes, whereas the SA-WM500 is about $150 and the PWS-12 is about $350.
but if Polk is better I'll go with Polk any way