I am installing my new PA660 polk amp in my boat and have a few questions. I am using speaker level inputs for my 4 6.5" polk speakers. My head unit only has one set of preouts, and I may add an amp in the future, so that is the reason for using the speaker level inputs. The speaker level input harness on the amp has all 8 wires for the 4 speakers, and there is one additional black wire. The manual says its a ground. Why is there a ground there? It also says the amp will auto sense the signal from the head unit and turn the amp on when needed, so I will not have to hook up the remote wire??