The PS-024 uses a pair of Toshiba M5218L chips (the same as those used in the Boss HM-2 and Boss FZ-2) and the PS-040 uses the exact same circuit board with upgraded components to create a heavy distortion with plenty of gain. This circuit then feeds directly into a very useful 5 band EQ that acts as a tone control for the distortion, essentially giving full control over the distorted frequencies.
These are truly outstanding heavy distortion pedals.