while using a cheater plug can, in theory, be dangerous, the *path* for ground woltage is generally dissapated thru the interconnects. using a cheater plug is basically dangerous when the equipment it's used on is not connected to anything else. i've had hum problems in the past, and i've tried the suggestion to hook wires between the chassis' to no avail - perhaps cuz they're awreddy connected, w/the interconnects? re: using a circuit thester, this sounds like a great idea, as ewe seem to be having the problem regardless of what combination yure using...
doug s.