Maybe stupid idea, but what do you think, to insert the passive tone control at the end of the circuit, right before the 1/4 output? I guess a true bypass tone control.....The buffer is active so it pretty much defeats the passive tone control purpose. What you can do is plug hot wire and ground of a PU (disconnected from preamp) to an amp and see what comes out. You may also try to connect the phantom coil. If you are happy with the result it is certainly possible to add a switch and a tone control to make the bass passive.