I reviewed the schematic for the Black Dog and I do not see a simple way to replace the passive tone stack. The main reason is there is a final MOSFET gain stage in the Black Dog that is followed by a 2nd order low pass filter. These contribute to the overall tone of the circuit. I fear that removing the existing tone stack would adversely affect the tone. You may wish to add the tone TwEQ to the beginning or the end of the Black Dog circuit. You may find that that would give you even more control over the resulting tone. Try it and decide before you drill the enclosure.