A beta-test version of my Treble Peaking Filter Calculator is now on-line (graciously hosted by dianemariekloba.com)! What it does is calculate the resistor and capacitor values required to make a high-pass shelf filter with specified turnover frequencies.
If you use this in the early stages of an amp or distortion circuit, before any overdrive or clipping actually occurs, the concept (and circuit) is essentially that of preemphasis like the RIAA equalization used in record cutting. In amps & overdrive/distortion, the idea is to reduce the bass frequencies that tend to flood the distortion stages.
Somewhere later in the amplifier system there needs to be complementary deemphasis. This often happens in the tone stack, speaker and speaker enclosure.
