I do not know a lot about stereos but would like to solve this annoying "thump". I have a Kicker 12 run by an SX6503. They are within 12-18" of each other. The tower and interior speakers are run by a KX8504. There is a sub volume control and the "thumping" gets louder as I turn it up. The only other items in the boat when on is the depth sounder and XM radio. Head unit is MB Quart. Annoying as .... Any suggestions??