My half power circuit is VERY unique. It does not alter the High Voltage on the output tubes plates , or the output transformers load, that is why the tone does not change , and why it will not harm the amp to toggle the switch with full power on. I do it all the time, that is the only real way to test it and to hear the reduction in power not the loss of tone. But like all things be responsible , this is a delicate expensive instrument.
Shoot toggling the Stand-By switch repeatedly on any tube amp can blow a fuse under the right conditions.