FIf you are working with the "new" power supply, the one with opamps, then each opamp compares the reference 12V voltage (supplied by D1 or D2) with the portion of the output voltage from the R8/R7 or R9/R10 voltage divider. If the voltages differ, the opamp drives the pass transistor so that the voltages become equal.
With the 511/750 ohm divider, V(out) = 12V * (511+750) / 750 = approx. 21.2V.
If you want a lower output voltage, you can reduce the value of the top resistor in each divider (R8 and R9). For example, 499ohm instead of 511ohm would give you 12V * (499+750) / 750 = approx. 20V, 475ohm would give you 19.6V, and so on. Paralleling 2kohm with the 511ohm gives you 1/(1/511+1/2000) = 407ohm and approx 18.5V.
However, if you bias is too high, a better thing to do may be to carefully tweak resistors around V(be) multiplier (Q14) in the amp itself. Reducing R19 from 825ohm and/or increasing R18 from 221ohm would decrease bias. Be careful though - should R19 be missing, your bias current would be very high.