Switch between amps

I regularly use an Ayre VX-5 Twenty stereo amp to drive KEF Reference 1s, but would like to be able to switch occasionally to a pair of Cary 572SE tube monoblocks.

You need to be very careful to choose a switcher using on a fully balanced power amp like the Ayre VX-5 Twenty stereo amp, it must be able to switch both + and - speaker terminals! Some selector like the Bryston 2WSB WON'T work because the "-" terminals are connected internally.

  http://old.bryston.com/PDF/Schematics/SSB2_SCHEMATICS.pdf

In my case, an added complication is that the Ayre has provisions for bi-wiring whereas the Carys do not.

The solution is using 2 stereo switchers, one on each channel.
On the Ayre side, use the "R" speaker terminal for high and "L" terminal for low.On the Cary side, using a pair jumper cable connect the R and L terminal together.

It would need to provide some sort of load for the amp not used.

Yes, it's a good practice to protect an tube amplifier and some SS amplifier.
 

I doubt such switch is available, but I'm not sure.

That's the reason I built my own.
https://systems.audiogon.com/systems/8685 
High quality speaker relay is the key! I highly recommend relay with bifurcated contacts like the Amplimo LRZ.

edit: Mola Mola Makua preamp

I was looking at doing the same thing and wanted to get a preamp that had dual outputs. There is a thread I created that discusses this. It turned out some preamps such as the Mola Mola Tambaqui, Wyred 4 Sound, Atmassphere and a host of others support this. I connected the manufacturers to see if it was OK to use the preamps this way.

You connect both amps to the preamp at the same time. You would turn the amp you want to use on and the one not required off. I was planning on have 2 sets of speakers in the same room.

In the end I decided to get the preamp with the best headphone section and not the dual outputs. The new plan is to manually flip the XLR’s from one amp the other when I want to hear a particular speaker.

Yes, you can do this. The easiest way is to use a HIGH QUALITY speaker selector switch. Luxman and Van Alstine make these. I haven't used the VA box but it looks good. I regularly use a Luxman AS-50R for making A/B tests on speakers.

There is a current thread on the speaker forum regarding this very subject.

Warning: you are going to get a lot of advice and flack about your inquiry here. Apparently selector switches are the root of all audio evil. I would agree with that re: cheap selector switches but I have never noticed a degradation in SQ from a good box. Keep the cables from the box to the amps as short as possible and utilize the same speaker cable that you use for your speaker runs and you will be fine.