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As far as I can see the base bias voltage on TR6 and TR7 is governed solely by R26/27 and any base current drawn by TR6/7.
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I agree, only R26 and R27 affect the output stage bias.
If R27 has risen to 80 ohm from 51 then this is the cause of your problem.
correct bias voltage ignoring nbase current would be 51/2200 * 9 = 0.208V assuming a 9V battery
with values you have 80/2530 *9 = 0.2845V
You actually measured this voltage earlier and it was 0.28V with 9.7V battery.
This increase in voltage will increase current through transistors. This will mean they will start to get hot and then you enter a thermal runaway situation where less bias is needed etc so everything rapidly overheats.
provided transitors are not damaged changing R27 to 56ohm will give
56/2530 *9 = 0.199V which is very close to correct value. So fit 56ohm resistor in place of R27 and everything should work provided the transistors are not damaged.
Jeff