I have used some practices similar to what's been described in this thread. I think my initial introduction was a "Gain Staging" document from Rane.

To this day, I still find gain staging to be the most confusing thing. I cannot explain it to someone because I am not confident in my understanding.

If you set the levels by ear... Does the balance stay consistent as one rides the input volume up and down, or would it change as you changed the output going into your crossover then amps? Is a voltage measurement the most accurate way to measure the outputs from the channels? And I've read people who say leave amp gains maxed, some others do not.

Is there a hard and fast rulebook to go by for gain staging that answers all this? Or is it too gear dependent to be that simple?