Now, I'm very curious about the design process of such an output stage. Is there info anywhere about how to do it? I looked up "hybrid cascode push pull" but I didn't find anything close to this specific topology.
Compare it to playing and writing Music.
I doubt there's a book on "How to write Rolling Stones songs" (not holding my breath, given their popularity there *might* anyway) but there are tons of "general purpose" books on different aspects of Music, including chord combinations, rhythm analysis, probably explaining Rock/Folk/Country/whatever types of music, etc.
Here you will find general use Electronics book, plus some dealing specifically with Tube Guitar Amps, but I found the latter always missing something and in any case not going end to end from a clean sheet of paper to a glass and metal chassis on a bench, where you plug a guitar and a speaker and play.
So the proper method is to learn the basic Theory, experiment, see what Commercial makers do , how and why, design your own starting simple and advancing bit by bit.
Eventually your own designs will start to work better and better

Personally I can design a cathode driven power amp, with Tube/MosFet/Bipolar power devices, just with the proper datasheets, go figure.
But it took me many years.
Now, *explaining* how to do that might take a couple posts, but *teaching* how to , so you can do it
fully on your own, end to end, starting with an idea drawn on a napkin and ending with a working amp, requires the full course, because that's what's needed.