Slap me if I'm wrong here, but isn't the common (-) supposed to be connected to the ground? That would mean that ifyou measured across the common and ground, you should get 0 volts as it's shorted out. If you get a voltage then obviously it's not right. There could just not be a ground wire attached, as well/ The last owners of our house upgraded from 2 prong to 3 prong and never changed the original wiring itself.
ONE THING'S FOR SURE, ALWAYS WEAR RUBBER SOLED SHOES OR YOU ARE AT MUCH GREATER RISK FOR BEING THE SHORTEST PATH TO GROUND! Obviously, you were. Considering the amp is probably isolated from ground by the transformer (s), your problem may lie in that.