Your test hookups are proper.
In my opinion, the metal case is supposed to be either doubly insulated Class II or connected to earth, otherwise it will fail the codes. It means you should not be measuring any voltage on the case. Stray Ghost voltage maybe.
There are few situation;
1. The DC power cord may be multi wires and one of them is earth wire which should be connected to the casing. The wire is broken. Or the earth is not true earth [earth broken somewhere else].
2. There is a short circuit in the internal circuit hence you have some problem with the amplifier. The shorts leak to case.
3. Cut exposed wire is touching the metal case.
4. [Append] Faulty power supply.
You should decide "finally" decide if your want to return it or troubleshoot yourself. If you do, I trust a lot of good members here could advise you how to resolve it and repair your amp.
The first thing is seek for any schematic of your amps, just to be sure.