Strings, strings, strings.
Still want flats but less boomy-y with loads more mids and upper mids? - TI jazz flats.
Then there’s rounds but heaps of difference here. My favourite ‘dark’ sounding ones are GHS Blanced Nickles.
Or go all the way up to steels and discover that your tone knob actually does something. Steel wounds on a P works for Lee Sklar, right?
I’m really into the idea of matching the right bass / pickups combination with the right strings.
Getting your low end under control onstage is another part of the equation, mind.