There's no reason you couldn't use a simple ABY. The tiny instant that the amp's OT isn't connected to a load while the switch changes throws isn't going to hurt anything. The real problem I can see where you technically shouldn't be using a guitar signal ABY is that the wire between any jacks and the contacts in the switch might not be quiet as thick as they should be as most pedal hook up wire is 24AWG and speaker wire is usually a much thicker gauge. Anyhow....you definitely do NOT want to use the AS&SFXR as a cab selector. It's only for instrument or line level signals. You want to use an ABY that is entirely passive.
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