A switch box would have as bad, if not , worse circuits than your receiver. There are some very expensive ones that are said to be high quality but it's still another object in the loop.

I think you have a grasp on what G's set up is, but, if you bought a preamp with the unity gain or "HT bypass", it would be designed for your purpose and you wouldn't have to mess with volume control.

For example, my set up used to be for 2 channel my cdp and turntable went to my Krell integrated which had the "theater through" feature. My DVD and satelite went into a surround processor. The front L/R outs of the processor went into the Krell. When watching HT, I just pushed a button on the Krell which allowed me to use it's power section as a slave to the processor. Either way, passive or HT bypass, would work for you. I'd hate to see you forget that the volume was all the way up sometime when you crank up the ole stereo side though