Have seen this passive tone control circuit in several guitar effects pedal circuits. I have added input and output buffers. The dc bias passes through the tone control circuit to provide bias to the Darlington buffer, as the base current is miniscule due to the very high beta of the Darlington pair there is very little voltage drop through the tone control for the dc part. As this is a passive tone control there is a certain amount of loss in the a.c. domain ( the actual signal )
