Switch your tone control around so instead of having the pot in series and the cap to ground it's the other way, and assuming a 500k pot you need about 3nf cap so that when you turn the pot to max R the roloff's about 100Hz, you wouldnt wanna go much higher than that.
If you're using a switch its very simple.
Chose scale of R in the 10^4-10^5R orders, and plug your resistor value and cutoff into this:
1/(2*pi*R*f) and it'll give you a capacitance value.
Your cap will be in series with your other pot when you engage it, and in s/p with your picks so your cutoff won't be exact what you wanted, probably a little lower.
Just buy a bunch of them round that sort of value and see which you prefer.
Alternatively you could use a voltage follower but you'll have to hook up a battery. This would be the cleanest, most precise way though and would be like having a TS in your guitar lol.
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