If you're looking for a big, crisp slap tone on the TRB you could try a Seymour Duncan preamp. The only thing you have to watch out for in these cases is corrupting the integrity of the midrange sound. SD can be quite 'brash' sounding to me in some basses, whereas say, Bartolini or Glockenklang might sound a little more culutred and balanced. Ultimately though, I think you could be barking up the wrong tree. TRB's, and especially the high end ones, are some of the best sounding 'production' basses you can get. If you want better you'd probably be looking at spending some serious cash with a custom builder.
This thread has got me wondering how good a TRB would sound with an ACG/East preamp though!