I think the first step is to evaluate how it sound when not plugged in and especially to compare it to the unplugged sound of a bass that you do like the amplified sound of. If you put your ear against it while playing and it sound sort of thin then, then spending more money on changing it is probably not going to be that productive.
For what it's worth (and it probably ain't worth much), I don't care much for these early 70's Fenders. I remember them being really heavy (ash) and the thick finish. They're worth a lot now because the older ones sell for so much more, but it seems to me that Fender had it nailed in the 60's and lost their mind from the 70's to the late 80's.
To me, these 70's Fenders sound sort of thin and cold. I'd be willing to bet that you Series electronics are telling you a truth that you'd rather not hear. Changing pickups may shift the balance a little, but I doubt that it will be a transformation.
I wouldn't be all that concerned about the pickup position or previous routing. Part of the Alembic Sound is due to pickup positioning, but it's going to be dominated by the active electronics, thru-neck structure, and laminated neck. Moving the pickups will make a change in the harmonic structure, but it will be relatively minor. Since we fret the necks to play, this means that the optimum tone on your bass just happens to be at a different fret.
Routing a lot of wood out of a body can change the tone a lot too, but since you've always had this bass this way, that's a non-issue.
I don't think you can do better than a set of Series electronics on a bass you love, and you're lucky to have them. I have a lot of crazy basses, but one of the craziest is a custom Modulus 12-string bass (4x3 strings, a la Cheap Trick). It's totally nuts - thru body 35 scale with a double thick quilt maple top with a lot of carving. I had wanted to have Modulus build it with a set of Series electronics but that would have driven the crazy cost into the definitely insane range (the electronics set alone was more than most people would spend on a fine instrument). I also thought about sacrificing one of my Series bass' electronics for the project but that was too painful to think about. I still wonder what that would have sounded like...
So, it seems to me that you better sell this bass of yours. And when you're ready to do that, please don't forget to send me your address so I can haul it away for you.
David Fung