Arkless Electronics
Trade: Amp design and repairs.
If it's a normal interconnect (earthed at both ends... as they should be) then it's nothing to do with the interconnect. FWIW I sometimes have to lift the earth on my own St20 to get rid of hum, depending on what it's connected to and whether or not an earth loop is formed etc.... at my own risk (slight as it is). To do it at YOUR own risk simply open the mains plug and disconnect the earth wire then reassemble the plug so the earth wire is hanging out and the plug top clamps it there... or insulate it using insulating tape... but make damn sure the earth can't move about and touch the live!! Usually one of the two conditions (earthed/not earthed) will be silent in any circumstance of particular other items of equipment in use. A St20 in proper working order is silent in use.