I managed a guitar store from '77 -'88, the peak of the "brass is better" craze. I put a lot of brass nuts on guitars and basses for lots of customers, and put on on my '73 Precision. And just being brass made no difference on any of them. The reason so many people thought that a brass nut made a difference is because in almost every instance, it was the first time they'd played their instrument with a properly cut nut. To this day, most nuts on factory produced instruments are cut woefully wrong. Almost always too high, frequently cut with bad angles to the strings, slots the wrong size for the string, etc. In my experience the only companies that ship new instruments with the nuts cut correctly (as in I don't feel the burning need to fix this freakin' nut) were Guild, Ovation, Takamine, and Lakland. Every other new instrument I've picked up needs the nut worked on.
So they get a new brass nut, which includes a complete set up and new strings- the instrument feels better, plays in tune better, and sounds better because of all that. And it's all attributed to the material the nut was made of when the real improvements are the proper set up.
John