Re: Need JB and Jazz with more fatness...
a couple of ideas...
1. As others have mentioned, maybe an alnico 4 magnet would do the trick. You can get them at Allstar Magnetics, along with alnico 2, 3, 5, 8, and ceramic bar magnets. That would be the first thing I'd try
2. Try a Rio Grande BBQ in place of the JB. It has less output, but it has a nice bass and midrange response with softer highs. I agree with Lew, a '59 would be a bit fatter sounding than a Jazz.
3. Install a Clapton midrange booster kit.
Ryan