These are mods do nothing for the treble content of a fuzz, they just let more or less bass INTO the circuit. While often a useful way to dial in good fuzz, a tone control after the fuzz is what allows one to control the treble of the fuzz, because distortion generates harmonics and treble, even when fed with a muffled, bass heavy signal.
With the Fuzz Factory, I've tried a big muff style tone control, and Mark Hammer's Stupidly Wonderful Tone Control, and preferred the latter much more. It knocks the output level down a fair bit, but it still was loud enough to not be quieter than the bypassed signal.
Here at AMZ is an article that shows the SWTC and a couple variations, the one at the bottom looks interesting because it offers treble boost as well as cut: