See that little blue square thingy with the really teeny weeny little round white thingy in the middle of it that is on top of the big black rectangular thingy?

It may not look exactly like that but possibly similar (or not), and it could maybe, perhaps (or not), be a small pot buried on the circuitboard somewhere, too, if it is not an encapsulated preamp like this Bart preamp. Can you Post-A-Pic® of yours (probably no need to remove it from the bass, or, possibly yes, if the pot is on the "dark side" of the board)? Problem that might arise: there may be no pot at all (goes along with the above mentioned phrase in quotes).
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(Some knobs have been removed for clarity, although they have nothing to do with the topic at hand)