Yea, I figure 15's are most efficient as far as what the specs say on most brands of 15's Just have to see how many I can run and then decide what subs.
There's videos on youtube of some blow-through setups. I know a guy from MS Audio that has 4 DD 3515's in a blowthrough on little power, 6 12's in a blow-through on like 4kwatts, 6 15's, 2 15's etc...