hi,
in fact it seems like everybody thinks you need a lot of distortion to get the jj sound, but no. I tried a few things, and what really works it to get the right frequencies to distort before the others. JJ used a hiwatt guitar amp in the early days : this had 2 essential features :
1. tubes
2. guitar amp high-pass filter in the preamp stage
Result: low frequencies are rolled off right in the premap stage, so the midrange is the frequency range being distorted before the others, especially with a p-bass having more midrange than treble. All this supported by enough still undistorted bass frequencies, that make for a robust muscular tone.
So, the midrange is being overdriven by the preamp stage, thereby generating overdrive type of distortion adding harmonically rich treble range (it wouldn't work with say a j-bass that has more treble in the signal, or any other 'hifi' bass having lots of treble right out of the pickup : that would lead to a hissy overly trebly tone). And the speakers that seem to work best are 15'', with that throaty midrange voicing.
Nuff said - if u're tired of my technical blablabla, here's what you need :
- A decent P-bass
- One Powerful valve amp (I got an Orange AD200B mk3)
- A preamp with "the right bass rolloff setting". Key is to hit the amp's preamp with this rolled-off low frequencies. Don't dial too much gain in, just enough to get that midrange peak overdriven and turn up loud.
- A nice big old school 2x15''
...there you go with that awesome viciously growling testosterone soaked Fender Precision tone ;o) Try the intro to hanging around - spot on !
Cheers then, and have fun !
Zak
PS:
1. JJ the man himself says that Rotosound strings, a pick and the way he hits the strings are essential ingredients of this tone. I can agree on the hitting strings part and the pick. But about the strings I'm more perplex, for me what works best are bright flats. yes yes

)
2. some of the Tech 21 VT bass pedal settings also approaches this tone, with not too much drive setting but high character (which, then again, is pushing midrange)