A room that size, assuming it's reasonably well sealed off, could be a good fit for something like the SB13U. You could easily get extension deep into the infrasonic range (think 10Hz) which isn't typically feasible with a vented alignment, even one as capable as the PB13U. However, if you were interested in pursuing that route, I'd highly recommend looking at the PSA XS30; a pair of those isn't a heck of a lot more than a single PB13U, and one to one should have an output advantage over the SB13U, albeit at the expense of size/aesthetics and some of the options available on the Sledge amp (which may not make much difference if you utilize a modern receiver with room correction).
