check your amp for a "polarity" switch on the back. if there isn't one there, try turning the 2 prong plug 180 degrees in the outlet. before the days when 3-wire plugs became common for guitar amplifiers (hot, neutral, and ground), there were circuits using a resistor and capacitor from what was supposed to be the neutral of a 2 wire power cord to the amp chassis. when 2 amplifiers using this arrangement are plugged in with one plug reversed, there's a 60-120V difference between the two "grounds". because the resistor was something like 470k and the cap was 0.01uf, the resulting current when you get between these two "grounds" (by touching the guitar strings, which are connected to the chassis of one amp, and then touching the mic case, which is connected to the chassis of the other amp) was usually pretty small. best bet is to replace the power cords with 3-wire cords, so you have real grounds for the chassis, and isolate the "neutrals" in the amps from the chassis (remove the RC network). the "polarity" switch on many amps just switched the RC network between hot and neutral. when i was in a band, i would spend about 10 minutes cross-checking amp grounds with an AC volt meter to make sure everybody's amp was not going to be a shock hazard.