If you can scope the signal on the "ground" then your scope is not grounded to that ground, I'd bet. You scope is maybe grounded over in the corner to the chassis or something? Between the ground end of the pot and the chassis , there is resistance. Wherever there is resistance, the potential for potential difference exists - voltage, and that can incl;ude signal. There is enough gain, especially in the dirt channel, in this amp and others for cross talk to skip stages even if controls are zeroed.