Repairing/understanding this guitar amp master stage
By measuring the voltage drops from Base to Collector on Q9 and Q10, I determined that Q10 was massively faulty, with a ~0.3V drop in both directions (swapping probe polarities). Because this was causing a short, it massively unbalanced the rest of the loop causing a great load across Q12 and Q13 and resulted in my documented hum. I replaced Q10, and noted the DC voltage present at Q12's emitter was now ~0V.

Through great interweb research I have discovered this amplifier arrangement is called a "Quasi Complementary" amp.