bugremachine,
You are probably right about the rca ground being the same as the chassis ground. But my personal experience has not been good when I mixed the chassis ground and the signal ground. In several cases I got hum.
My solution has been and is now to keep them separate. Perhaps some time, I will check it with an ohm meter.
Question: Are you also referring to the ground on the AC outlet being the same as the signal ground?
ozzy