Playing with flatwounds is a big part of the battle. I know from reading about James Jamerson that he had a few particular elements that contributed to that Motown sound.
His '62 P-Bass had the bell with a foam mute and he never changed his heavy guage Labella flats (the exact same strings as Pino). Those two elements right there helped to fatten his sound. He played with his tone and volume on the bass all the way up and, for live gigs, used his amp EQ to set the treble at half and the bass as high as it would go. The Motown guitarists hardly ever used amps in the studio, though, so Jamerson would just plug into the console and turn his volume up just past the point of overdrive. Then it was up to the engineers to set limiters and equalizers on his signal to keep it from overpowering everything else in the mix.
You'll never get it quite perfect (even the Motown guys seemed to lose the magic when they moved to LA), but that should point you in the right direction.
