Why do you need a switch? Your preamp has two pairs of balanced outputs. You can connect both amps to the preamp, and simply power off whatever amp you're not using.
Balanced switch for preamp output
My Ayre KX-5 Twenty is connected to a VX-5 Twenty amp that drives a pair of KEF Reference 1s. I'd like to alternate to a pair of Parasound JC 1 monoblocks driving Magnepan LRSs. Can anyone recommend a passive balanced switch?