Me and my friend just installed 2 subs and an amp into his crown vic. He has a kenwood head unit that only has a bass adjustment, it doesnt have a separate subwoofer adjustment so you can only adjust the bass on the whole system not the speakers and sub separately. It sounds kind of weak, but I know the amp and subs could be a lot louder without getting distorted because we plugged my alpine head unit into them that has a separate subwoofer control and it was like 10x louder and still clear. So my question is, is there any way we could boost the signal to the subs, tell them to be louder, without him having to buy a new head unit? Some kind of signal booster or something? Thanks in advance for any help.