FWIW, I just worked on one of the big Sansui switch boxes that you'd find at a dealer "back in the day". They use surprisingly complicated switching. When you start pushing any button, it opens a separate relay to disconnect/mute. I think it may also connect to some power resistors so the amp always sees a load. Though the speaker switching is fully floating, it appears all the negative speaker leads are common. Everything in it, in terms of switching, is custom. If I were trying to duplicate the function today, I think I'd go with relays and a PIC or something to insure the correct break before make behavior and loading while disconnected. I'm not sure about the loading as there weren't enough large power resistors for all the amps. There were some smaller ones that may have been just enough to insure stability on tube amps, maybe 50 or 100 ohms or so. Naturally there was no manual or schematic available. Regular switches make sense for a few amps, but not for ten or so!