This is a Little Big Muff going in a Morley PWF (from 1975) through Mesa rig:
Youtube link
On the Kill 'em all Anesthesia version, Cliff had a Big Muff after the Morley with the Fuzz from the Morley on:
Youtube link (last example)
Cliff used the Mesa D-180 (not the Bass 400) it (they, there are two of them) can be seen in this pic:
His normal tone is pretty gritty, I asume he used the high gain input of the D-180 (I never played one of those)
Youtube link, Cliff Isolated track for Fade to Black