I have discovered that if the outer aluminium part is being touched it increases the hum. My assumption is that the aluminium part is/or should be “grounded” to the amplifier via the six screws?
Another discovery is that when doing a short circuit of the RCA input the noise in the RCA switch selection lowers to nearly nothing.
I have tried to see if the internal cabling is correct, and it seems like. It though wonders me why the audio cable runs directly under the transformer.