I had the same bass as you and fitted a Nordstrand MM pickup to it. Over £100 - so it's got to be good, wouldn't you think? Well, no. The Wilkinson that was in there originally sounded a lot better to my ears. I swapped back and forward several times (and tried series/parallel wiring on them both) but ended up selling the Nordstrand.
So, try replacement parts by all means but bear in mind that there is a lot of marketing hype surrounding pickups and preamps - just look at the Duncan Antiquity range to see what nonsense goes on. This is why these companies can charge up to £200 for an item with a parts value of less than a fiver. What good aftermarket pickups do give you is consistency, i.e. you pay your money and you can be reasonably confident you've got a good one.
Do buy used, because this could otherwise prove to be an expensive exercise. I'd go for the preamp first if it were me: getting the right amount of bass boost on that bass is key to getting it to sound good.