When you switch-in the extra output there is no noticeable noise, clicks or sound other than volume change
Cathode lifting would be almost completely silent. There might be slight rise in plate voltage on a meter for the B+ since the overall plate current drops, but since the plates are still loading the OT, it's not like just pulling tubes.
which outputs and where to check and I'll let you know...
The lift would be on the ground to cathode resistor, the "input" of the power tube.
Thanks, I won't pretend to understand all of it - or have the need to. I've heard of a couple of guys installing their own "half-power" switch to a Hiwatt but got the impression it was something you had to decide which mode you'd be in before firing it up.
Just for paranoia, I'd do it in standby, but I've accidentally switched the power (it's a knob on the back next to the DI level knob) on my Trace a few times with no ill effects at all.