Help deciphering the bd-2circuit

[Teddeeh]

So im finding it hard to get a breakdown of the bd-2 schematic. Can anyone explain the various areas to me? I am looking to tinker with it and i think i know what i want to do but im a little weary of tinkering with one further at the moment until i can get a little help.
One of the thinks i want to do is boost the input signal, and swap out some of the transistors, which i already have done with q9 and 12.