I have a subzero ported box with 1 Concept 15 in it. The box is 1.8 cubic feet inside volume. What would be another good sub I could replace the one I have now with to get more db's? Remember all I have is 1.8 Cubic feet. This is if I am not satisfied with the outcome of what I have now. I'm bucking for at least 147 to 150db's.