Buzz, it looks like you have received a lot of very good advice. I have extensive filter experience in my years as an electrical engineer here at Neptune audio. (BTW, I have always loved Elliot's website
).
I didn't see it mentioned, but here's what I would do. If you notice, the rolloff of the L-R filter is steep @ 36db/octave, but the actual attenuation @ 16Hz is only about -3db. Also, there is some small droop in the passband. I would suggest a higher Q filter, like maybe a Q of 2-3. This will give you a peak in the response where you undoubtedly want it (in the 20-30Hz range), and a much faster initial rolloff.
For your convenience, here are the formulae for calculating the F3 and Q of a unity gain sallen-key filter:
To calculate the frequency, take the two frequency determining resistors, multiply their values, and take the square root. Use that as R, and calculate the frequency using the formula: 1,000,000/C(in uf)/R/6.283.
I use one million as the numerator because caps are usually in microfarads. To get Q, find d (or damping). The grounding resistor on the non-inverting input is 2/d. The feedback resistor is d/2, so the ratio of the resistors is 4/d^2 (four over d squared) with the larger resistor being the grounding resistor.
Say you want a Q of 3: d=0.33333 (which is 1/Q). d squared is 0.11111, so 4 divided by 0.11111 is 36. The ratio of resistors is therefore 36:1. To find the resistor values for given caps, take the ratio (36:1) and take the square root (6). Multiply R from the above equasion by 6, and that's your grounding resistor, and divide R by 6, and that's your feedback resistor.
BTW, the 741 opamp is passe. The TL072 is far superior in every way, except driving capacitive loads. Be sure to put a 100 ohm resistor or larger between the output and the world. Happy building!!!
Ken