They started with the '70's DOD 250 Preamp OD and then put every possible type of clipping diode type in it.
From the very first transistors ...Germanium
to the Silicon to a modern FET even an LED.
If nothing else you can hear the difference each type makes
with all else being the same. The pedal makes a great reference tool.
Oddly the warmest is a tie between the germanium and the
You have gain and a level control for each.
I find the concept exciting but there are many OD's with just
a single choice for clipping that have a more desirable voice
and with at least some EQ to shape your tone.