Polk PSW10 is a great little subwoofer. I have it under my WOZ. When I bought my Metallica Premium, I happened to have a leftover Mackie subwoofer from my old audio/video production studio. It barely fits under the cab and holy crap...when I hit the spinners I can feel my legs vibrate. Is it overkill? Absolutely! But why not put a spare subwoofer to use!
So what I'm saying is, get a subwoofer hooked up pronto.