Audio amplifier schematic explanation
It's a standard differential pair. The left transistor takes a signal from a previous stage and the right transistor takes a fraction of the output signal and creates negative feedback to control the gain of the amplifier.

Here's an example that uses NPN transistors as the differential pair: -



Your circuit is made slightly more complicated because of the Baxandall tone control circuit sat right on the right transistor's base which, is essentially doing this: -