Martin wrote when sharing the original schematic at Freestompboxes:
Alright, I got it finally figured out... an SHO-style MOSFET boost with NO CRACKLE!!!
I came up with a new gain control for it that allows for the same range of gain adjustment without having to adjust the Source. Since the Source stays fixed, there is no crackling.
While thinking about how to solve this problem, I realized I could treat the basic gain stage as an inverting opamp. So, I set a fixed max gain and tried a variable resistance in series with the input and voila, works perfectly.
Check it out and give it a try; it's a great improvement over the SHO and it's easy to tweak the sound to taste.