Question regarding geofex simple parametric EQ

[dune2k]

Hi,
I play bass and since my current amp doesn't have a parametric eq I plan on building one in stompbox format.
I stumbled on R.G. Keen's article regarding a simple parametric eq and that looks like what I want.
Now the question: I don't really get how to calculate the capacitors (C1/C2). 100Hz is a little high for a bass guitar, so I'd like to shift that down to around 30 Hz. But, except trying lots of different caps, how to find out the caps values?
I tried to understand the formulas given, but I don't understand them properly...

[PRR]

If you want 3 times low, make caps 3 times bigger.

I strongly suggest you aim at 50Hz, not 30Hz. Bass rarely goes below 42Hz. Speakers almost never reach below 50Hz. Boosting subsonics just makes slap and distortion.

[Mike Burgundy]

I think a mistake/typo crept into the given formula.
It should be F0=1/(2*PI*(SQRT(L*C1))) or in other words F0=1/(2*PI*(SQRT(R1*R2*C1*C2) (because L=R1*R2*C2)
Then it's just filling in the numbers, R in Ohms, C in Farads.
R1=470
R2=51k to 1M (actually 1.051M)
C1 and C2 determine center frequency.
Doubling them would mean (R1R2C1C2) increases 4 times (2 squared), the root of 4 is 2, so the entire equation indeed halves.
.33uF and .047uF yields 60-260Hz. I'd go for something like that. Remember the frequency specrtrum boosted or cut spreads out from the center frequency F0.
I play with a low B (31Hz fundamental) but that is a barely audible frequency, especially at volume. What you hear is the harmonics, a bass doesn't put out a sinewave, more of a sawtooth, so there are lots of harmonics there. I have designed and built cabinets that have an F3 (-3dB lowest frequency) of 30Hz, but that's mostly a "because I can" excersize. It will rattle the glasses on the bar, but it doesn't really sound lower.