og high gain would have to be hendrix, because fuzz slammed into+ cranked preamp + maxed out power section, that's three layers of saturation, but blocky.
next would be the SD-1 + JCM800, which would be the first cascading gain, roll your own type.
then we move into the cascading preamp gain era, where we move all the gain staging into the preamp (mesa)
i think of high gain as a saturation that can sustain a palm mute (ola englund), and likewise is meant to sustain leads. where you palm mute and the pick attack is distorted but the decay is not, that's my threshold for the beginning of "high gain".