hey! I allow myself to dig up this post because I have the same problem. I use the FX Loop in stereo with a Y cable for the SEND and Return L + R. When I plug my pedals in series everything is OK but as soon as I want to use the loop it "hum". That it is with 6 pedals or just 1 ... The cables are handmade and the power supplies are voodoolab. I really have trouble understanding the problem ... A conflict with the ground?