Nah, it's your bog standard passive low pass/high cut filter. The internal one is just more of the same in series. It can go lower but both are just 6db per octave subtractive filters. You're not gonna get any more bass, you can get less treble which may translate as more bass, but if you want more actual bass, it'll have to come from elsewhere. However, it's likely to change the tonality of the pedal to bring in more bass earlier in the circuit that may result in "flubby" drive. The coupling cap to the second stage of the opamp would be one place to try as well as the cap to ground in the feedback loop of the first opamp stage. But since were just talking about adding capacitance, you can always piggy back more caps on the solder side of the board, put them on toggles if you wanted even. I'm not looking at a schematic so I don't know what part numbers, but you should be able to look at the schematic and figure it out.