I find it best to run an amp up on a DBT before working on it just to see how it will react. I will also take baseline measurements with a scope of power output, noise, wave shape and whatever else before going at it with the iron.
Not always possible of course but it's good to know. Some amps don't react well to reduced voltage/current and it's nice to know that before pulling your hair out after you've had it scattered all over your bench.