The "classic" JPJ sound is a Jazz strung with Rotosounds going through an Acoustic 361 rig (one 18" speaker in a folded horn enclosure). Check out the Led Zeppelin DVD set for some shots of his gear. Later on he switched to Alembic basses. Currently he uses Manson basses into SWR gear.
As far as how to copy the tone... first, check out how he plays the bass. Nothing on the amp can make up for how you actually play the thing. Second, tone is about midrange management, tweak the mids, cutting the frequencies that mask out what you want to hear. To my ear, JPJ's early tone was missing a lot in the 1khz range, later he had a more hifi tone. DON'T ADD A LOT OF BASS.
I've had a few...sorry if I'm incoherent.