Well the HDC315 and 18 have basically all of the same parts with the same power handling, hell the 18 even has the same power handling as the HDC310. Might not get as low, but my buddy has the 18 in a cherokee and I get louder, its just when we stand outside of his car hes shaking the earth.
UPDATE: I will take trades for, some 6.5" comps but you have to have an amp to power them that your willing to throw in. I will also take HU'S mostly ones with screens.