The KoT / Wampler Pantheon style presence control is a pot in series w c8. So you could switch in resistor there for higher presence. Or, subbing c8 for a different value would change the cutoff freq of that filter, adding a resistor in series keeps the frequency the same but adjusts the slope that it rolls off, as I understand it.
Or subbing c6 works, but note that your tone pot is already adjusting the cutoff freq of that filter, so the tone pot and switch would be very interactive, whereas c8 is a separate filter.
In either spot 0.22uF seems pretty drastic considering the math, but then again the ears have the final say...