The newer Seymour Duncan STC-3A and -3P preamps are very similar, but they are not plug-and-play compatible with the wiring harnesses. They require installation of the new controls (included and prewired) and, to retain functionality of the separate slap switch, rewiring from the included volume push/pull switch to the original. This might be the best bet, since you're stuck buying it with new controls anyway you might as well use them. I just haven't figured out whether to go with the 3A (active) or 3P (passive) setup. The only real difference is the blend control pot, which is 100K for active and 250K for passive. Most likely, basing the correct preamp on whatever value the original blend pot is will be sufficient.
But, if you want to completely upgrade, I would suggest spending less on BassMods. You'll likely end up with two stacked knobs (treble/bass and mid/passive tone) plus an active/passive mini toggle. Pros: Run passive or active and still have some tone control, without modifying the look of your Jazz 24. Cons: Eh... you'll have to decide that for yourself.
