Q6 and D2 are a current mirror. the current through D2 is mirrored by Q6 to drive Q9 symmetrically to Q7 (i.e. whatever Q7 is doing, Q9 is doing the opposite.). Q8 and D5 set the bias for the output transistors (Q14, 15, and their drivers Q12,13). Q10,11 are protection transistors and their purpose is to shut down the bias and drive signal to the outputs. have you replaced R24,25? if they're open, Q10,11 could be saturated constantly because the slightest amount of bias to the drivers and outputs has nowhere to go except to drive Q10,11 into saturation instead of flowing through the low resistance of R24,25 and the speaker. that would also account for the lack of bias and Q7,9 running so hot. R24,25 are the "emitter resistors" for the outputs (which are arranged as "composite" or Sziklai Pairs, Q12,13 act as a high beta NPN and Q13,15 act as a high beta PNP), and if they're open the voltage sampled by Q10,11 will be too high, and activate Q10,11. normally Q10,11 are in the "off" state, and essentially out of circuit. R20,23 sample the voltage across R24,25, and activate Q10,11 if the voltage is too high, shutting down the bias and drive to the output devices.