Hi bob540, sorry for the late response because i didn't check this page frequently.

I constantly switching gears in my setup for experiment, it is a mess that nobody like to see!

Thanks, @jond

That is one of the few units I custom built for my friend to demo amplifiers in his shop for his customers and his audio club.
His customers usually don't like to wait an hours or two for warm-up/setting up and compare tube amps, so these selectors can switch between amps, while one amp is playing the other amp still on with dummy load connected, it is the best compromise we can think of.