Warning! There are lethal voltages inside your RM20. If you don't know what you are doing, then get a qualified amp tech to make these mods.
- lift the right side (looking from the front) of C9 from the board;
- solder to pin 3 of an A1M pot (1M log / audio taper) pot;
- solder an 1nF cap from pin 2 of the pot back to the vacant hole from C9;
- ground pin 1 of the pot (a convenient ground point is the ground pin at R20;
- drill a hole in the back of the head for the master volume pot (somewhere around the serial number is a good location). Alternatively, on one of my amps I've relocated the boost pot up a little and put the master volume below it so its on the front of the amp. Relocating the boost pot does involve cutting the PCB though.
Explanation: we're basically bleeding signal to ground before it goes to the power amp.
Here's what it looks like if you mount the master volume pot at the rear of the amp:
In case its not clear from the picture, the 1nF cap goes to the middle of the pot (pin 2) and the 0.1uF cap goes to pin 3.