a single 15" in a ported enclosure will rock and you will only need a net internal volume of 3cu/ft. I have run the gamut of subs and for small spaces it's hard to beat a single 15" or a pair of 10's ported of course with a 30 Hz tuning frequency. Crossover at 50-80Hz.
Jeff
how do the 15s do in the Sound quality department, getting loud is important but it doesnt matter if it sounds muddy.