Your talking about an experience I’ve had many times in the past 40+ years of buying , flipping and selling instruments, As a matter of fact its more often than not this happens. You have an idea in your head as to what the bass should sound like, and then it’s hugely disappointing when you plug it in.
The even harder truth is, you’re not gonna fix it. Now, don’t get me wrong, you can upgrade an instrument that has the fundamental sound you want, but needs just a little tweak, ( electrics usually), but if the sound is just fundamentally poor, all the tweaking in the world ain’t gonna fix it. Roger Sadowsky has stated, and truthfully so, an electric bass is a first and foremost an acoustic instrument. If it doesn’t sound good acoustically, it’s not going to sound good electrically amplified. No matter what. I’ve lived that experience too many times. Lesson learned? Get rid of it. Fast. Even if it’s gorgeous, plays really well, etc. If the wood ain’t good, move it.Maybe somebody else will dig the sound. But a piece of crappy wood is the turd that will not shine. Ever.
Conversely, if the fundamental of good sound is there, meaning, Its sounds pretty darn good, but plays poorly, or needs a slight change in EQ, then by all means, Mod it. Fix it, set it up, restring it, all the things suggested. But if the acoustic sound is poor. There’s no fixing that.