Back in the day, I had the same issue with an amp. It had a two prong A/C plug, and turning the plug over, i.e., reversing the prongs, solved the problem.
I guess since only one prong was supposed to be driven and the other was supposed to be a ground source, they must have wired the ground source to the chassis ground. So if that prong was plugged into the driven side of the socket, the chassis wouldn't be properly grounded. Turning the A/C plug over was the fix.
Your new unit probably came with a three prong plug that can't be turned over. Maybe the internal wiring of the A/C connection needs to be switched.