We've talked about this in another thread. As Hal has mentioned, check for AC voltage between Earth and chassis. But also look for DC volts. You said that the amps have two wire cords and plugs. Are they polarized or non-polarized? If it is non-polarized, for kicks and giggles, take two sets of voltage measurements, one with the plug reversed. My concern is that you are dealing with a component designed around a "hot chassis". Many designs save weight and money by skipping the power transformer, and going straight to a rectifier. The negative end of the bridge rectifier often becomes the chassis ground. The current that might be able to flow between chassis and earth is better expressed as rare, medium rare, well done, rather than leakage. (does wonders for scope grounds)
If you are dealing with an amp with a hot chassis, plugging it into an appropriately rated isolation transformer, and then earth ground referencing the chassis, is the only really safe bet. I know that this could get a little costly but might solve several leakage and hum problems as well.
Joe
