The Tempest Amp Distortion is a combination of the Friedman® BE-OD and Dirty Shirley pedals, two very similar circuits that were designed to emulate high-gain amplifiers.
It’s not a true amp emulator of the sort that replace the tubes with JFETs, as Runoffgroove is famous for. Instead, it’s fully driven by op-amps, but very carefully designed in a way to mimic the response and character of a Friedman amplifier. And by all accounts it has succeeded, quickly becoming one of the most revered “amp in a box” pedals on the scene today.
The original Friedman BE-OD is missing a midrange control, while the Dirty Shirley has a midrange control but converts the Tight knob into a toggle switch. The Tempest project combines these together, preserving the midrange control while also leaving the Tight control as a knob instead of a switch, for a total of seven knobs.