IIRC they use their own pre-amp
IMHO and IME any preamp can only alter the tone that is already there (to various levels, ways. extremes), so if the F-bass tone is what you are after, I'd suggest that you start off with pickups that provide a J-ish tone (I call it "modern vintage") and then try to move on from there.
Just a suggestion YMMV etc etc
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