I have a (passive, separate power amplifier, cross over in my preamp/processor) ported sub, currently limited so it doesn't bottom if (when) it receives subsonics (like those present in the limiter configuration test tones on my preamp/processor). Getting rid of the lowest frequencies and raising the limiter to just shy of amp clipping seems a better idea. Contrary to urban legend, my BFD has no problem passing tones < 20Hz, so that doesn't work for me. Fortunately, I'm a professional geek with soldering iron, box of passive components, and a couple of opamps. A 24dB/octave filter is trivial, but I haven't a clue as to how I should pick a -6dB point. Suggestions?