Generally, buzz near the nut IS the nut heights, buzz in the middle is truss rods, buzz at the high end of the fingerboard is bridge height. It's never quite that simple as these things CERTAINLY overlap and interact in strange combinations. But assuming you have no high frets or a wierd jumble of curves in your neck bow/relief, this will give you an idea of where to begin looking.
I also can not stress enough what pilots learn: Your eyes will fool you, and most of us just don't see 1/16 or 1/32 increments with the 'naked eye'. If you get even a cheap 6 steel rule marked in 32nds, you'll be shocked at the difference in heights, clearances, etc., you'll find in your action. And NEVER do any of this with the bass laying on its back on a table: Put it in your lap.
You want the string clearance under each string at the first fret and the last fret to be uniform, to match the curve of the fingerboard. In real life, most tend to run the bass side a bit higher than the treble side: The bigger B's and E's just move in bigger arcs than the G's and C's. We're aiming for a consistent feel across the fingerboard.
If memory serves, relatively speaking, flatwounds are the highest tension vs. round or 'ground' wounds, so I would think you'll need some tension on your truss rod nuts. Leave them loose and the neck is just going to wander around and make you even crazier chasing this!
Check for roughly the same clearance over the first fret on all of your strings. Then do the same at the last fret. Then adjust your relief if need be, and raise the bridge slightly to kill any remaining buzz. In a perfect world, we'd play perfectly straight necks: In real life, we shoot for dead straight and let in enough bow/relief to eliminate buzz and to feel right to us. I can feel too much bow, other people remark how strange an almost straight neck feels.
No two basses will adjust exactly the same, and no two players will be happy with exactly the same setup. You work to find YOUR setup.
J o e y
