If your cables and amp are fine like you have indicated, you may need to check the solder connections on the analog output for that channel or check to see that there isn't something else wrong with the connector on the G68.

Try taking a short piece of wire and touch the ground side of a known good RCA channel to the affected one to see if it goes away. Also, does it continue to hum with no other source equipment connected to rule out a ground loop hum?

Reflowing the solder joint may take care of it.

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