You can go with different subs, but integrating them can be (considerably) more difficult than integrating idential subs.
Do you want sub 20hz bass? OR are you ok with above 30hz and strong 40-80hz output? I ask because the PB-1000 has sub 20hz capability and it is very linear but it is not an output monster, BUT the PA-150 is an output monster in the midbass area and is within your price range.
The Outlaw Audio LFM-1 Plus is a very solid choice ( I have 2 of them ) BUT they are $620 shipped, unless you wait til they are on sale for $499 shipped which is what I did. And I would honestly scratch the BIC F12 and Hsu VTF1 off your list.
I would sell the Polk and buy the SVS PB-1000. If you can see your way to spending more, you can buy a higher model. Or, if you can afford it in the future, buy a second PB-1000.
It is also good to keep things in perspective. You are not going to get dance club bass from a single subwoofer in your price point. Especially in a room your size. You will likely want to try putting the subwoofer in a corner to maximize what you can get out of it, or, as has already been suggested, do near-field placement, if you are only going to be listening to it in one spot.
You might also want to read this:
Projector: Epson Home Cinema 2100; Receiver: Sony STR-DN1080; Speakers: Monitor Audio Controlled Performance Inwalls and Inceilings; Subwoofer(s): SVS PC-2000; Bluray Player: Sony BDP-S6500; Remote: Simple Controls; Cables: Monoprice
my room is large like yours too, they and the more experienced guys on this site really made the difference, although I am still tweeking. good luck!!