Bump! Long time Mesa player, I received my first PRS amp today, the MT15, and I'm having the hum issue. For mine, it's not a tube, it's the effects loop. When you plug a guitar into the input with nothing in the effects loop, the amp performs perfectly. The clean channel has zero noise. However even putting a patch cable in the effects loop elicits 60Hz hum immediately. Running anything in the loop, pedals, rack effects, other preamps, or just a cable = 60 Hz hum. It's not a tube or the transformer, otherwise it would be making the noise with nothing in the loop. So is there a fix for this? The amp sounds amazing and this is a fix that would make the amp nearly perfect.