For $500 the SVS PB-1000 would be good. For a little more, about $600, the Rythmik LV12R would be nice as well but it is a little larger than the PB-1000.
Listen to Cel, he makes a very important point.
I took a serious look at those and from everything I saw they perform just as well as the box versions and cost less. That would be a lot better choice for your room than the PB-1000. I'm assuming you are talking bout the PC12-NSD. Like I said, with your room I would only get the PB-1000 if you are planning to get a 2nd one down the road but you just can't stretch the budget right now.
But if you are willing to stretch the budget to $700 then that really does open up more possibilities. The PC12-NSD would be a good sub and another really good one would be the HSU VTF-3 MK4. There is also the Outlaw LFM-1 EX at $650 (and Outlaw has Black Friday sales from what I understand).
And if you will, allow me to muddy the water even more and toss out one more sub suggestion just so you know of its existence. The PSA XV-15 is a 500 watt 15 inch sub that is very powerful and a lot of bang for your buck. Its $800 shipped (I know that's $300 over your original budget, just letting you know about it is why I bring it up).
Ultimately, with a room your size, I think you need to seriously think about going dual subwoofers down the road to get the best bass you can in a room that size. But start with one and if that makes you happy then you will be all set.