just like imtjnotu said but in different words: your box is the equivalent of the transmission in your car. you can have any motor you want in the front, and any goal for the car that you want, but only ONE transmission is actually THE perfect one to put all those parts (motor's power curve, road/tire friction coefficient, vehicle weight, rear gear ratio, effective tire diameter, suspension loading etc) in harmony with one another to accomplish the goal. any other gear ratio/clutch-flywheel/torque converter combination would come out less productive. your sub is just a motor with a power curve and that box makes it do what it can in that particular vehicle.