Lack of shielding is a possibility, but also poor earthing practice.
Wire has a small but significant resistance, and if you daisy-chain the earth connection from the input socket through the amp to the power supply, those small resistances can cause problems. Better practice is to pick one earth point, eg the power input socket, and run a separate wire from the amp, the input socket and the output socket to that point. It's known as star point earthing.
David