I'll also relate the history of these insanely knobbed basses, as told to me by Ron Wickersham:
He developed this filter (essentially a state variable filter, so it can be band pass, high pass, low pass, or notch) to build a box that could be used in the studio. I think he made one for Wally Heider and for a few other studios around the Bay Area. Phil and Jack just loved what it did for them and wanted to buy one, so then, in the heady style of the day, they decided it would be cool to be built into the bass itself, to allow them to have them at their fingertips regardless of what amps they were playing through.
To provide a little context, they had experimented with building preamps into instruments prior to this, but they were simple buffers. They discovered that the beloved BiSonics had a superior frequency response to most pickups out there at the time, but even with a low capacitance cable, they'd lose an octave with 3' of cable. So, the preamps were built onto the bottoms of the pickups. This idea was later taken up by Bartolini and then EMG and active pickups are now a garden variety type of pickup.
People sometimes think that sound of the state variable filter can be recreated by parametric filters, but they are really different. State variable filters require a bit more thought and planning to get a certain curve, but they also provide a more organic tone. It's pretty easy to imagine how dealing with the filters in a psychedelic state might be challenging, so set it and forget it is a pretty good strategy, especially considering how much tonal variation can be had out of these basses from physical changes, such as pick attack, where you strike the string, pickup balance, etc. I use my Series II lowpass filters all the time, in conjunction with outboard EQ's, so I think those circuits are a great compromise.