Ernie, I'm going to respectfully disagree re impedance matching vc's vs. a selector box.

I'll try and be brief. The objective is to protect the amplifier.

With impedance matching vc's, you set those to the appropriate setting. It increases the amount of impedance per vc, whether the speakers are on or off.

With the selector box, you set it to increase impedance, yes, but when a room is turned off at the volume control, no additional impedance is on the bus.

Essentially, if a person uses impedance matching vc's, they are loading the amplifier more with speakers off then the same situation using a selector box. To my mind, that means that I'll get better sound quality with a selector box. The great Eric Johnson, now with Universal, explained this to me years ago. If it makes sense, credit to him; if it does not, it is my interpretation.....