Jeff,
That's the same train of thought I had going when I tried to use the GCX for switching preamps, because each loop is true bypass also. I used the sends to feed each preamp. The problem is the way the relays in the GCX work. when they go to take a loop out of the signal path, the relays short to ground. This is where the hum was coming from. It's not an issue when using any of my ADA rack effects in the loops, or using the loop sends as a momentary switch in the effects to bypass them, just when I'm feeding the preamps with them. The GCX even has buffered guitar inputs.
Currently, I'm using A/B/Y switches to switch to different preamps because I'm leaning more towards a 3-system guitar rig, but for my large rack, I'll probably use the Ground Control MIDI switch and bring in the preamps into a Rane SM-82 mixer that way. Program 128 in each preamp is a dead program, so everything is set to "0" in the preamps for that setting. The Aux Sends and Returns in the Rane has the dbx Overeasy noise gates which applies to all four preamps so I won't have to use the built in gates.
The outputs of the mixer feeds the stereo effects chain, then goes to the power amp and speakers.
I hope the looper box you have doesn't operate the same way as the GCX, or you may find yourself getting the same hum issue I was getting. I hear you loud and clear about the cables, the GCX is the same thing