If you are sure that it's not a short in your guitar or cables, ( and it doesn't sound like it is because it's fine with the effects loop active ) then my best over the internet, can't see it, guess is that it's a switching jack in the effect loop, probably the send jack, you need to take it to a good shop, strangest part of this is that both amps are doing it, you must have some bad mojo working,,,,,,,.........................