Bryston 2WSB switch box to choose amp that drives speakers

I really want to be able to choose between a low power amp for tv/background music and my good amp for serious music sessions. I'm tired of amp rolling, have posted recently about this and got good advice not to use two sets of speakers cables but only one live amp. So I'm thinking of a device like the Bryston, but have the speaker outputs be the amp inputs and the amp input be the speaker output. As long as only one speaker is selected at a time...electrically that should work. My only concern looking at the schematics is that the red positive leads are wired together and the black negative leads are switched. Could this still cause voltage to be applied to the unpowered amp? If so would swapping polarity work? There's a schematic on this page.
Any help on this would be greatly appreciated.
LOL I could use more exercise but I switch back and forth so often that it's a real pain in the arse, to the point where often I don't even bother connecting the good amp. The reason I do this is because the good amp runs in class a, so it's very hot and draws a lot of electricity, plus its a vintage Threshold that I want to limit its use for music only. I can't justify watching the news or football with it in the audio circuit. I could just use the tv's speakers for that but I also like the little amp for background music, I'll leave it on all day while coming an going. I won't do that with the Threshold. 
Good catch about the commonality of the red terminals, Treynolds155.

There are some circumstances in which that switchbox would work ok in selecting between two amplifiers if all of its red terminals are treated as black and vice versa. But I wouldn’t feel comfortable doing that with your Threshold S200. It’s manual, which I found at, states as follows, in reference to the S200’s speaker outputs:

Each channel’s output connector consists of a red and a black post on a common base.

These outputs, should be considered as electrically isolated from the chassis and from each other. In the case of dual channel amplifiers there should be no direct interconnection between the output terminals of the two channels under normal operation. Connect the output terminals of each channel only to the loudspeaker, not to each other or to anything else.

I would infer from that that the black output terminals of your two amps, as well as their red output terminals, should not be connected together.

Other conceivable issues are the power handling capacity of the switchbox, and whether the switch is break-before-make or vice-versa. If the switch is make-before-break, when changing its setting the two amps would be connected together for a brief instant, which conceivably could be a problem if either amp is powered up at the time.

Best regards,
-- Al
Here’s a basic selector,

You could also look for a Niles SAS-1 or a Niles SPK-1. These are no longer available new but can be found used. They would offer options for switching automatically.

As I posted this, I just noticed Al's post. Hopefully he will see my post and comment if my suggestions are suitable.

Al I really appreciate your looking into to this to that degree! This is really quite maddening that there isn't a quality switch that can accommodate it. I mean really I'm just looking for a four pole double throw switch that won't muck up the sound. I'll end up making something if I have to.