Dead tone on P/J 79ish

Hi there!,

First time poster, long time reader.

So I've had this bass for about...10 years, [its a handmedown from my dad].

It's a passive fender P/J setup from 79 onwards. The bridge and a few other components have been changed in its history so its a bit of a hybrid fella. [Its also left handed]

Now my dilemma is that simply put, it has a nice warm tone but
1. About 2 weeks after replacing a set of strings the tone DIES completely down.
2. The tone afterwards pales in comparison to what I hear on modern basses.

I do think it is a nice sounds but it's not what i'm after..so my situation is basically, is my bass salvageable?, or should I look into buying a new instrument.

I'm an avid slapper and this is the main reason it doesn't quite cut it. The tone of the bass reminds me of some Early Graham stuff in sly and the family stone. Sort of flat and dead...works for graham but With busy music its much harder to cut through without sounding to twangy!!!

Anyway Essay complete

do I get a new bass and perhaps try active pickups or do I stick with what I have and give it an upgrade of sorts.

Cheers Mike

[Also im left handed and live in Australia, so trying new instruments is a major pain in the arse]