I think you have the Polk PSW10 which is their entry level sub. This is not a very good sub even when you talk to the Polk guys on their forum.
How much do you have to spend and are you willing to go used?
Sunfire True subs kick butt. I now have two of them. I have one and just bought a second to put in my HT. Though I might try two subs on my 2 channel system for a while and see what that sounds like.
If you go used, you can get a great deal. I just snagged an original Sunfire True Sub for $440 shipped. I also have a Sunfire True sub MKII which I got in the mid $500 but it was local so I did not pay shipping. There are several out on eBay right now which might drive the price down a bit but we are close to Christmas which the last two years drives the price up.