a/b/y with phase switch and ground loop

[eniacmike]

I tried to make a buffered ab-y switcher and it worked well with low watt amps in the house, but when I tried it with two marshall half stacks I had really bad noise and ground loop issues.

http://www.geofex.com/FX_images/humfree2.gif
http://generalguitargadgets.com/projects/15-boostersrouters/16-amp-aby-switcher-box

I came across this and it goes about half way to where I want to get which is basically the radial bones twin city buffered ab-y with phase switch and a ground lift. I am assuming I can put the ground wire to the B output on a spst switch just to cut it, but I am not sure how to do the phase flip. I know from my car stereo days you can wire a sub backwards flip the + and - to flip the phase, but this doesn't work with unbalanced guitar lines.