What you need to do, is hand draw the schematic from inspecting the circuit. Then redraw the schematic a few times till it starts to look like a published diagram. This will teach you more about amps than book reading.
That's the best advice.
I learnt about amplifier design at 14 years of age, not from books, but by stripping and repairing. The art of tracing out the circuit diagram cannot be over-emphasized.
The limiting factor with tubes is that the people who held the knowledge are slowly passing away, and as you all know, today it is MOSFETS, IGBT's and normal bipolar transistors in most cases. As you know, most low-cost guitar amps use amplifier integrated circuits to do the job.
