Always power down everything before changing anything.

It's all very straight forward, but always do everything cold, never with power on.

That said, it is most important with replacing high voltage things, like tubes, for instance.



That came from my old electronics teacher, WWII veteran Navy communications officer who could seriously do or make any damn thing you could imagine with tubes!

True about the tales from just messing with signal cables, but I suppose it also depends upon how much gear you have downstream from that particular cable switch.

Going further, I would never introduce any piece of unknown gear to a set up without not only powering down everything, but also bringing gains down to nil, when possible.
Then powering up all the gear in a sequence that kept the main power stages at low output until all the gear is stabilized and all your connections have been confirmed and then maybe double confirmed.

I have some stories, too.

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