Sam,
Look at Jaco. He played a Fender Jazz, and there's thousands upon thousands of them out there, fretted, fretless, active, passive, etc. I've yet to hear someone sound like him. Same goes for Stanley Clarke, or Chris Squire, or Geddy Lee ...
It doesn't have to be expensive to be good in the hands of the right player. A quality bass helps, but it isn't the determinant in the end. Ability, skill, talent, feel, etc. is.
All this being said, I do feel a quality instrument helps one play better simply because they are more adjustable, easier to set up, etc. One can do the same with a less expensive instrument, but it takes more work and patience. I know my Alembics help me play better, as does my Musicman Bongo and Fender Jazz.
Alan