I did the mods on the Bass Muff last year.
I bought the Black Russian Muff yesterday.
They are that close.
If i remember correctly, R42 affects only the boost funktion.
Pushes it from a bass boost to LPB1/HardOn like boost.
And C8 seems to be part of the low pass filter of the tone stack.
Today i will find some resistor in R42 instead of the capacitor
i used to fit in.
Did you like the Bass Muff for guitar?
This makes a better low pass filter than any Treble Booster!
Excuse me, i soldered 1k to R42 instead the 39nF.
I will find a solution for C8 too.