Polk Audio PA660 amp questions

[rb8484 10+ year member]

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??

Thanks

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[naughtyca 10+ year member]

you can still use your pre out and if you added a sub amp you can either connect to the pre out of the sub amp to the pa660 or just get a y connector and split the signal from that 1 rca and let your amp do your crossover frequencies.