My friend has a 2000 Ford Ranger extended cab. He installed his system in it last night 12' Comp MTX sub in a sealed box and a Kicker KX1000. With both doors shut and windows up the sub sounds like it is bottoming out and is not very loud at all. The amp was set with a DMM also. It sounds like this at lower volumes too. However when he opens both doors it does not sound like that and it gets at least 3 times louder.
What is the cause of this and is there any way to fix it?