It is common in the USA/Canada to have a two prong plug on things like audio amplifiers. The metal chassis is also commonly connected to the neutral (should be ~= ground) with a high value resistor in the megaohms. If the plug is reversed then this will connect the chassis to live instead. In that case, the current would be limited to a nominally safe value but you might feel a buzz/tingle.
Does your amp have a polarized plug (one wider prong)? If NOT then reverse the plug and try the test again. If so, then check whether the outlet is wired correctly. The narrower prong is the live, the wider one is neutral. You should measure around 120 V between live and ground, and around zero between neutral and ground. Please use a safe multimeter and safe technique when measuring mains, and avoid ever measuring anything at the panel.