my head unit only has a mono subwoofer output and i took it to a car audio place to get my two 12" rockford fosgate subs installed (they are in a box). after the guy was finished he said he had to connect them to the rear speakers and not the subwoofer outputs. the subs do not hit as hard as they should. is this because they are hooked up to the rear speakers? how do i make them hit harder?