I am fairly certain the thump is the amp powering down. When I was in the military I built a killer home stereo piece by piece and it would thump when I would turn the power off. A guy I work with has a Harley F150 with aftermarket subs and it does the same. I think most amps power down when the vehicle is turned off or music source is turned off as not to kill your battery. I would not fret. If you cant help but fret, professional installations are usually garaunteed. My80 k-5, I had best buy put a sony xplode head unit with xplode 6x9's. When ever I had periodic problems(like in line fuse and such) they would fix it free of charge. BTW I cut the speaker wire to sport bar and spliced in 6x9's. The carpet box they are in slid between rear seat and ext wall snug enough they arnt flying around. My head unit slid right in and plug matched and everything(it had an older sony head when I bought it) Things never work this easy for me. Anyways improved sound with little hastle and not 1 dollar spent. I will get crazier with my tunes at a later more expendable point. Priorities must take president, such as rust proofing.
