I installed the master volume last night. It wasn't working well at first, because I had wired pin 1 to the solder lug that R28 was connected to. When I turned the master volume down, there was still a lot of volume coming out of the amp and the useful range of the master volume control was really small (the volume was full on with master volume above 2 or 3). I tried swapping pins 2 and 3, but that didn't help. Then I realized that there was a 56K resistor between the solder lug and ground, doh! Flipped pins 2 and 3 back to the original configuration, wired pin 1 directly to ground, and now everything is working well. I need to play around with it a little more, but my first impression is that the master volume is a nice improvement. There is a big difference between tweaking the volume at the send, return, or master volume stages. I need to play around with it a lot more to see what sort of settings work best.