I can't help but recommend what I just bought,
the Earthquake SuperNova MKIV 10. For up to $700 you may even be able to land the dual 12' model, but I doubt you'd need that much bass.
Search for it on eBay and you should pull up plenty of options in the $350 range.
It's quality has totally blown me away, it's relatively inexpensive, has a small footprint, and if you go with the cherry finish it has a much higher WAF (if that matters) than a black box sub.
For under $400 with shipping you can't go wrong! Oh, and since your room is even smaller than mine, 15-20 Hz shouldn't be a problem (well, for the neighbors maybe, te-he).