I've given up with expensive upgrades.
There's a general concensus about what should, and shouldn't, be good. Certain standard bridges that people swap for other bridges as an upgrade, certain body and neck constructions that are better than others, a pecking order of parts if you like, to achieve that holy grail of bass attributes, sustain.
I recently bought an early 70s Kay bass, widely regarded as an utter piece of shite, and not without reason, a plywood neck screwed to a thin plywood body. The original bridge had been replaced with a cheap BBOT with the wrong string spacing. I fitted a Squier Bronco bridge as it had the correct spacing, these were routinely upgraded as they were considered a very poor design. I've refinished it with several thick layers of poly which should dull it's tone as well.
Out of the 15-20 basses I own the Kay has the longest sustain and is the most resonant, it rings like a bell when played unplugged.
This makes a mockery of all the accepted theories of bass construction and hardware.