or he could do what the guy said above and just get 2 12 L7s and just go all out on an amp that has plenty of power and he could just keep adding subs if he really needs more bass..but 2 or 3 L7s should be f***in loud.
i got 4 L7 12's dual 4 ohms with a kicker kx 2500 it sound's good but i think more power will be better. so hit them with 3000 rms that will be some basss