Thanks for the response!
Everything was bought in Germany, so 230V with 2 pin plugs.
I tested with different sockets and flipped the cables around, but to my ears nothing changed, always remained the same volume and tune. Also when I plugged the CD source back in.
Are there any components that are prone to fail in these units? I read in several places that capacitors might go bad with age, could this cause something like my problem?