Sorry for the late reply, but with the Big Muff it helps to replace more than just the input and output caps. The circuit has coupling caps all through it, to isolate each section from DC.its great, not quite as big a difference as with the wah, but its much more responsive and it doesnt get too farty sounding.
If you look at this schematic, then the caps you would want to change would be C1, C3, C4, C7, C12 and C13. Raise the values of these and you'll allow more low end through the circuit.
You could also increase the value of C6 and C9, but this might make it a bit more muddy sounding as it will mean more lower frequencies will get clipped.
The BMP is a good circuit to replace all the coupling caps with sockets and experiment.
You can also mod the tone stack for more bass response. I put a switch on mine to switch between the stock tone stack and a bassier one, however unless you also raise the value of the coupling caps then your bass response is limited there.
For playing around with the tone stack a great tool is the Tone Stack Calculator.