Actually Jamerson's bass was usually recorded direct. He had an Ampeg B-12 as a studio monitor, but the bass you hear on the recordings was DI. And likely through a tube compressor. The Ampeg B-15 as part of his tone is a myth, but he did use one for live gigs.
Not to interrupt (and fan) this flame bait in the making, but Ampe
x was the tape machine, not Ampe
g, the bass amp, which according to just about everyone but the guys who wrote books about it decades after the fact, WAS used to get the bass sound on those old Motown tracks. Funny that a Precision through a B-15N 'just happens' to sound exactly like those old Motown tracks.
"Well informed sources" cite everything from Fairchild limiters (which would have been strapped across the main mix bus but never dedicated to an individual track) and tube preamps in the tape machines while conveniently overlooking the fact that the tone is easy to cop; the magic is in the playing.
