I ended up putting a 500k trimpot in place of R12. I wanted to find the nearest common resistance value that produced about a 25% increase in maximum volume to my ears. I found that 330k would be a good value for replacing R12 to achieve this goal. I am really doing this because I couldn't get enough volume increase during solos at live venues; I use it as a clean-ish boost pedal. I have some gigs this week so I will be able to try it out and report back. It could be that I am just hitting my amp too hard on stage and it's just compressing more without much increase in volume.