Ok so I put the bass flat on my lap and tried 2 things :
- pull just a bit towards me (or push away from me) the string (between tuner and nut), being careful to not increase the break angle at the same time, in order to simulate a narrower nut. Buzz gone.
- push downwards a bit on the string to simulate a higher break angle. Buzz gone. In this scenario however I can't guarantee I'm not also stabilizing the string side-to-side at the same time, so I'm not 100% sure that this is the problem.
I will try to shim the strings anyway to see what that gives.
But yeah, maybe I put the strings wrong.
The open A and E did not buzz before the setup though, but maybe something was hiding it somehow.
I normally put the ball into the bridge, then the string through the tuner and twist from there.
This time however I put the end of it into the tuner after having bent and cut it, since the bass came this way and in the DW videos they are also put this way, but this is new to me (it also seems to me that the chance of twisting a string is higher this way ?).
Also the bass came with around a single wrapping of string around each tuner, which is the fewest I had ever seen.
I just looked and now each string goes 2.5 times around the tuner, and the break angle is definitely higher now than it was before (and before had no buzz so I assumed it was good like this).