Most high gain amps with five preamp tubes get a big part of that from the preamp section. So saturation should be happening even when the master is low. Just turn the master low, are you still getting high gain? If so, that’s your preamp section, most of the time. Lowe power mode will allow more of that to come from the power amp section. But you’re still probably getting the majority of from the preamp.
Tone connoisseurs can tell the difference between power amp gain and preamp gain, but I’ve only ever really played high-gain amps regularly and played them at reasonable volumes. We have a vintage unmodified JCM800 which is a lower gain preamp design. And if you really crank that, with a Variac to lower the power output, you can get some early Van Halen-style saturation. But it’s crazy loud at that point.
Our main amp, the Hellcat, is a five tube preamp. There’s not a huge difference between the cranked master and the low master. It still sounds like our amp.