Many popular overdrives used a simple adjustable filter for the high frequencies. It was called “Tone” or “Treble” and really just reduced the high frequencies added by the overdrive electronics. Of course, this greatly limits the variety of sounds and possibilities.
After much sound testing, Kai Tachibana developed what was then, and we still believe, the unique double filter control called “Spectrum”. Not only the usual highs are raised or lowered, but also the lower mids/bass (~300 Hz). In the 12 o’clock position the sound is almost neutral. Turning the control counter-clockwise minimizes the treble and bass. Raised clockwise.
This allows a variety of EQ settings with just one knob.