I'd have to say playability first, then tone.
I have on more than one occasion modified a bass to improve tone. I have never, other than adjusting action, done anything to improve playability of a bass. I've improved tone by changing preamps, pickups, bridges.
For me, playability starts with neck profile and extends to balance, action and fret conditioning. Of those, I'd only feel comfortable adjusting action and maybe changing strap button position to adjust balance. Otherwise I'm talking to a luthier, or more realistically, looking for a bass that already has good playability.
Peace,
James