I pretty much always run both pickups on full and I use the tone knob to get the right amount of clarity, or definition, or whatever you call it for the line I’m playing... usually tone is about 1/2 way up for fingers or pick, and about 3/4 to full for slappy stuff. If I want the Jaco finger punch, I’ll pluck over the bridge p/u and it gets me in that world. I don’t solo the bridge p/u like Jaco though, the full blast “zipper” just isn’t my thing. If it’s yours, please by all means, zip away. I’d say 95% of my playing is over the neck p/u (and both p/us volumes on full). That’s just THE sound of a Jazz bass for me, and what I feel is the best sound I pull out of a Jazz for the majority of normal everyday playing.
I look at a P Bass as a simple, one trick pony because it is, and it does that one trick wonderfully. And I look at a Jazz as more of a 3 trick pony... neck p/u solo’ed is the ”P Bass impersonation”. It’s cool. I’d rather have a P Bass, but it’ll do. Both p/us is THE jazz bass tone. Period. (insert obligatory “your mileage may vary” here, as not to offend someone who feels differently about their Jazz Bass tone). And then there’s the solo bridge p/u “Jaco zipper”. All the different volume knob combos in-between are basically irrelevant in the mix of the band, to me anyway. It’s those 3 that pop out as the clear “tricks” that a Jazz Bass does... And for me, the middle “trick” is the only one that I really care to use. I love that trick.