Could stepping down from 220V/50Hz to 110V/60Hz cause issues with household electronics?
That is very likely the problem.  The stepdown transformers step down the voltage, but do not increase the frequency - that is why a hair dryer, for example, will operate slower on 220V/50Hz when it stepped down to 110/50hz.  The audio processing components of the receiver are likely depending on the correct input frequency.  Its also possible that you might have damaged the receiver from doing this.