The Seymour Duncan 805 takes its inspiration from a TS-808 Tube Screamer but is designed with an expanded gain range and adds three-band EQ in the form of three mini knobs for bass, mid and treble.
Balancing the level and drive knobs provides a whole range of boosted and driven sounds. With the drive knob at minimum, you still get 8dB of gain and can wind up the level to give you a useful clean boost.
At full whack, the drive knob offers 36dB of gain for a valve-like overdrive that can add a second channel to any amp.
At their (notched) neutral positions there's little tonal shift, but the three EQ knobs each offer cut or boost at a well-chosen frequency for fine tuning of the sound, particularly the crucial mid knob (operating at 750Hz), that can either scoop out the sound or give you a nice throaty honk.