I have some low output pickups on my strat- nothing too major, but I can only get a very teeny tiny amount of grit out of the fatpants. I get plenty when I throw my custom switching to middle+bridge in series. But any single coil modes are not quite gritty enough. Could this be achieved by lowering the value of R1? or should I try something else?
You shouldn't get any "grit" out of a Fatpants. It's clean boost, and even a high output guitar shouldn't be overdriving the single gain stage. If you're getting actual distortion, I'm inclined to thinks something might actually be wrong ... like maybe some mis-biasing is going on.
R1 is part of the biasing network and has a small effect on the tone. Lowering it will decrease the treble content. Slightly.
If you actually wanted to make the circuit distort, there are ways to make that happen (R5), but they're going to destroy some of what makes it sound special by changing the frequency response.
Are you playing through a valve amp? Perhaps what you maybe hearing is valve break up from the boosted signal going in to the front end of the amp..
George