You are making a big mistake and petertub already pointed at it. Muting relays and speaker protection are not novelties elsewhere in audio and they do prevent nasties like the ones you describe. In your case shorting the outputs is a way better solution than taking the risks of defects in the output transformers (which may occur when disconnecting the speakers!)
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This is the modern (post Morgan Jones' revolutionary article) thought about all snubbing around power transformers. It's always been the method for damping/terminating signal transformers, but took a while to get to power circuits.
The modern thought goes: dI/dt happens across the transformer winding, so that's where the snubbers need to be. Applies for the secondary rectifier-induced noise too.
All good fortune,
Chris
The switch, the line impedance looking from the transformer's primary (Includes cables, plugs,fuses, line filters, etc.) and the primary forms a series circuit. If we can neglect the line impedance, thus putting the snubber in parallel to the transformer primary and to the switch, is the same from the small signal point of view. But letting it across the trafo's primary avoids leaking when the set is off as I just said. You are free to put it where you want, at last instance, but assume the consequences by your own.
Another option, not yet mentioned, is a MOV across the primary or the switch, but MOV's sometimes catch fire without any known cause including not pass the voltage limit of the device and no current limit is offered to it. So, use it with care and ALWAYS use a fuse between it and the line.
Another option, not yet mentioned, is a MOV across the primary or the switch, but MOV's sometimes catch fire without any known cause including not pass the voltage limit of the device and no current limit is offered to it. So, use it with care and ALWAYS use a fuse between it and the line.
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