I have 3 guitars and 3 amps. Amps are Micro Terror, Hughes and Kettner TM5 and Marshall MG30CFX (my only combo amp). If I plug 2 of my 3 guitars into the MT, I get hum until I touch the strings. It hums REAL loud if I touch the pickup or adjusting screws. If I plug the same 2 guitars into the Marshall or the H&K, no hum at all. Now......if I plug the 3rd guitar into any of the amps, no hum whatsoever. Only the 2 guitars hum in the MT. I am not electronically inclined, but I figured it can't be a grounding issue or the 2 guitars would hum on all 3 amps. Any ideas?