The ground connection to the seat bolt is a poor return ground path to the battery for the amp.It's power is being pulled directly from the battery using 8 gauge wire. The ground is mounted to the chassis at one of the two rear seat bolts. The ground is using the same 8 gauge wire. Later tested it using a temporary run of 0 gauge straight from the battery negative.
You are relying on the several connection points as well as the size of the jumper that connects the body to the battery negative (ground) post.
What kind of operational results did you get when you ran the 0 direct from battery to amp?
Have you tested the voltages/idle symptoms with the amp disconnected?