My question is this: can you audition it before you pay for it (or, alternatively, can you return it if you don't like it)?
A buddy of mine has one of the lower end Polks. It cost him something like $150 full retail. A lot of people here would scoff at that sub, but it works well in his room that is near the size of yours and opens into the kitchen and living room. I was impressed with how it worked with the rest of his system. It's small, but it does the job when combined with his other speakers and room acoustics. His wife would kill him (and then me) if he were to buy a sub like mine because it's too big.
My point is that the sub might be a very nice addition to your system, regardless of specs and what other people think of it. However, it would be nice (IMO) to be able to try it out before committing to buying it.