When Alembic made your bass, they set it up for the environmental conditions and wood conditions at that time. Each piece of wood is different, so each neck's neutral condition (no strings on) will be different. When it's strung up and relief is set, they do whatever what's needed to hit the factory target for action.
But your climate and desired action may not be in the range where you can adjust to what you need using the truss rod.
I think the next thing you should do is find a good tech to take a look at the frets. Sometimes there might be some unevenness in the neck or frets that's causing your buzz. Your bass didn't have this problem when it came out of the factory, but can be the result of wear, wood shifting, or imprecise prior fret work. This kind of buzz is more likely to show up with light strings and low action (although I don't think you're target action is all that low) and the truss rod won't adjust it out.
Another thing that a good shop can do is to use a neck jig to force a little more relief into the neck, then tap the neck to see if the they can get the truss rod to shift a little (this works on guitars, unlikely they will have the right jig for a through-body bass!). They use a jig like this and a heat blanket to do the heat bend.
If you have a lot of fret height, a good tech can also mill some relief into the tops of the frets, regardless of the shape of the wood. This can be very effective for eliminating buzz when you want a very flat neck.
A bunch of stuff to try! I'd definitely pay a visit to a good tech first to see whether the problem can be solved in the frets.
David Fung
