This ones pretty simple if your module has the appropriate circuit board. If your module board has a designation of MODU_1-0 then you're all set. Here's what to do:
- remove C1 (this is the bright boost cap);
- remove R8 (this resistor prevents an annoying pop when switching the bright switch);
- add an appropriate sized resistor into R23 - you can start with something like 47k. The lower the value, the less gain there will be in the low gain (switch to the left) setting.
The resistor in R23 works as a "grid leak" or "grid load" resistor in the coupling circuitry between V1B and V1A. It provides a path for signal to ground. The lower the resistance, the greater the signal path to ground and the more gain attenuation occurs with the switch to the left. With the switch to the right, R23 is switched out of the circuit.