I had the same issue with my rig .... I use the M13 in the effects loop of my amp carvin X100B reissue. turns out what was causing the the noise was a ground loop that only occurred using the 4 cable method. I purchased a Behringer Hum Destroyer for 25.00 ran it on the return side of the loop out of the m13 going to the amp and the hum is gone . So a 25.00 hum destroyer and a 6 inch patch cable solved the issue for me.