Don't know if you're familiar with a gent by the name of Joe Gore. But he's a superb guitarist (played with Tom Waits and a host of others) and does the occasional column on modding for Premier Guitar magazine.
Anyway, he has a website (tonefiend.com) that gets into a lot of mods and sonic experiments that it may be worth to looking into. It's guitar oriented. But much that's there carries over to bass. And many of the comments are from fellow experimenters and tinkerers, so this is one site where the comments can actually be worth reading.
He did a series of articles (with videos) on some of the varitone type circuits you can play with. Search link to just the varitone articles Invalid Link Removed.
Cool site. If you're into passive mods, Joe is your guy. His "no talk with deadpan expression" videos where he demos things are a riot.