I don't know then. A lot of the motown vibe is in the total package, meaning the recording mechanisms they used on those old recordings, and all the other things. I've found that it can be difficult to get my basses sounding just like a recording, but then when you get on a gig it just sounds great. I covered "Signed, Sealed, Delivered" a few months back with a group, and it was just like this. I kept trying to get that same tone, and it was close, but it just wasn't exact. When we actually rehearsed the song for the first time, it sounded great, and was even better on the gig. And that was with an L2500. The SB-2 with flats would have take it even further.
My SB-2 has the dual tone modifications done to it so that I can roll of some highs right on the bass. This does seem to help approximate the motown tone when I'm just noodling around by myself at home, but on the gig I never seem to use it.
One thing that does work is to do what Rocco did, which is to fret the notes with your index finger, and then slightly mute the string above it with your other fingers. It takes the top end of and fattens things up.
Also, you could experiment with a thin strip of foam right under the strings, next to the bridge. You have to play with the size and thickness of the foam to get it just right, but the end result is really nice.