At the same time you switch the tube amp AWAY from the speakers, you MUST have a load online for that amp. Tube amps need a load.
If you intend to shut the tube amp off before switching, you will eventually forget or someone not totally familiar with this switch and system WILL eventually switch the tube amp away from the load and ZAP.
SS, doesn't matter, however I don't know about SS thru autorformers....but I suspect not.
Also, if you do some kind of DIY, you must use 'break before make' switches to keep from switching one amps output to the OTHER amps output, even for a fraction of a second.