FAC design

[Tage]

I've been looking to build an amp with an FAC control. I know how both Orange and Matamp did them, one right after the first gain stage and the other after the second. What I am concidering is the effect this control has with different, ie. larger, capacitor values. I was thinking, 100nf, 47nf, 22nf, 4.7nf, 500nf. I realize that these progressively reduce my coupling capacitor value, with some big jumps, but is there a way to determine alternatives to the original values? or is experimentation gonna be the name of the game?

[J M Fahey]

Calculate the crossover frequency and determine the effect it can have on your sound.
Switching from, say, 16 to 32 Hz will be inaudible; going from 500 to 1000Hz will be easily noticed.
I think original values must cover the practical range and were empirically determined.

[Ronsonic]

Capacitor Decade Box. Get one and experiment with different values. I'd expect they pretty much got it right on the old Orange amps. Those did sound good.