I've noticed a few things in relation to tone:
First, less is almost always more. Especially when it comes to your amplification's EQ settings. My dream is to one day do what Joel just described and never look back.
Second, remember where you started before you start fiddling; once you find the "perfect" settings, go back to "zero" and listen - bet you are either right back where you started from, or you like "zero" better.
Lastly, the more you "chase" it, the further away it gets - we have a local guy, good player, nice dude, has several of our basses. He has had a habit over the ten years I've known him (I've been here HOW long now?!?!?!

) of getting a new bass, playing it for a few months, then convincing himself he needs a different preamp. Getting the new preamp, loving it for a few months, then convincing himself to get another "better" one. Loving THAT one for a few months, then putting the original preamp back in his bass and realizing it was what he was looking for to start with. Now, when he gets a new bass, I just tell him "You've already put 3 different preamps in your bass, and you love this one. Right?"
The most effective tone control in your entire signal chain is always the same: your right hand.

Second one? Blend control between the pickups.