Motown was played (almost all of it) on a P bass with old flats. A J is a different sound - a good sound, sure, but not what you hear on Motown recordings. The closest a J can come to a P sound is with the neck pickup soloed, and.....that hums in a lot of places. The quintessential J sound is both pickups full up, which sounds quite different, and doesn't hum. Both basses can be thought of as having "vintage" sounds, they're just very different sounds.
I don't know why I assumed it was a Jazz bass with flats haha I've learned a TON of motown too and was very happy with the tone on my p-bass with flats…
