I put a couple of Potter DPDT relays in a box, wired to the usual banana-type posts and powered by a wallwart that was laying around. 8 feet of zipcord and an SPST toggle make/break the coil line to provide remote capability, but I think I'll increase the length to 15 feet or go wireless. The relays are 10amp @ 277vac, but I wired the contacts in parallel and used a relay for each channel so they could be separated in the box. I'll shoot a pic if you'd like, but there's really nothing to it.

When you can switch directly between speakers nothing can hide.

We also used it to listen to differences in speaker wire with limited success - you really need two of them to completely isolate the wire.

John