The SD-1 has a few other differences that could account for some difference in sound. If you're just trying to assess clipping differences, maybe try to find a v2 timmy demo. Similar gain stage to the tube screamer, but it has a switch for the clipping. I believe Paul C said up should be asymmetrical, middle is symmetrical with 2 diodes per side in series and down is symmetrical 1 diode per side like a ts-808.