It's magic!
Yeah, three volumes....
I don't know about the amp master vol and whether it's also profiled and has the same basic effect as a master vol on the profiled amp but the question has crossed my mind a couple of times.
I also don't know if the cab vol affects the entire amp stack tonally or not, it's nearly impossible to not be seduced by the increase/decrease in volume and believe there's a difference.
I just keep an eye on the lights.
The light! The light! Go to the light!