x2 Or any high powered 8 really (1508's, RL-i 8's, TB Ultra 8's, ect. ect.). Of course they wouldn't out due 4 15's, but you're not going to get 4 15's in a box that's 3.5 cubes max. Put 500W to each, and you'll be happy.4 SA-8's
4ft^3 gross volume isn't enough for two 12's.