The Zvex Super Hard On has become one of the sought after boost pedals today. Derived from the preamp section of a 60s recording console, the SHO uses negative feedback to control the gain (which is what gives it the crackle when adjusting the knob). It's capable of being a clean boost and adding some sparkle to your tone, or pushing your tube amp to break up and creating a great overdriven sound.
If you want to add a master volume control to it, remove the 47k resistor and use a 50k pot. Lug 3 to the Out pad, lug 2 becomes effect out, and lug 1 goes to ground.