Thanks for such a thoughtful and detailed reply. Makes sense. While I have you, a possibly related idea that turns up on forum posts about this exact thing...
Seems That only "half" the 12ax7 is being used, and if you want more gain you "just" wire in the other half - e g the 'spare triode.'
Sounds intriguing - and scary! In addition to mysterious. I know a fair bit about guitars but noodling around inside amps is not my thing. Ringing any bells?