There is more than R8 &9 in that string, it is R8,9,10, & 11.
Adjusting the trimmer R10 adjusts the Q3 current. Q3 conducts through D1 and R8.
How hard Q3 is conducting changes the voltage at Q7 base, and in turn, Q9.
The bottom side is the mirror image of the top.
As stated earlier, you will probably have to increase the value of R11 and R12 to get the bias right for the sub. output transistors.