OK, so now you can plug the RCA plugs into the Moreplay. I would put them in input #2. Start with the selector switch set to 1 or 3, then measure the AC voltage across each RCA jack on input 2. (Black probe on shell, red probe on the center pin for each jack).
If you still see about 0.8V, then put the selector switch on input 2 and recheck.
If you still see about 0.8V, then measure the AC volts between ground in the amp and the red wire leaving the selector switch, as well as ground in the amp and the white wire leaving the selector switch (ground in the amp is any black wire on either the balance pot or ground pot).
If you still see about 0.8V, then measure the AC voltage at each of the middle lugs of the balance pot with the balance pot about centered. It will be less than 0.8V.
None of these tests require the Moreplay to be powered up, and I'd suggest leaving the power cord unplugged for now since it just isn't necessary.
-PB