What JTUK said is more or less true in my experience apart from the preamps. You should firstly decide what sound you want and then search out an instrument that delivers. There are some manufacturers like Lakland who have managed to achieve an incredible level of consistency in their instruments but you won't get much in the way of customisation. If you go to a custom luthier you're more or less at the mercy of their wood pile. Finding a custom luthier who has nailed their wood selection is like finding a needle in a haystack. To have the best of both worlds, find an instrument that gives you the sound and then have it customised. You won't be able to get much back for it in terms of resale value but if its one in a million then its unlikely you will ever need to sell it. This does assume that you will have tried loads of basses before settling on what you're looking for.
Whether it has a maple fingerboard or walnut body doesn't really matter at the end of the day. Basses are like a recipe, in that they represent a mix of ingredients. One ingredient can be added or altered in such a way to compensate for the inadequacies of another. Whether you like the flavour is another matter entirely.
I've tried loads of basses over the years. The ones that stood out for me were made almost exclusively from maple, but a particular combo of hard and soft maple. So I have a couple of Spectors now but they've been heavily customised to make them sound warmer than standard Spectors.. I also really like graphite necks but haven't found a bass yet which combines low action with warm sound. I'm still working on that one.