The Marshall is one of my favorite Tonebender circuits - and, modded to use the leaky Russian GT404s, it's a beast. But... I'd like to tinker with the circuit to see if I could get a greater 'sweep' with the filter control.
Marshall Supa Fuzz
It appears that the filter control is connected to the 10nF cap in row B and the 4.7n in col 16 and controls how much is getting dumped to ground.
To increase the range of the sweep, is it as simple as increasing the 10n (or both caps) - or would other mods be needed?