Cyril

are you sure it's not the trussrod?
what happens if you (gently) hurt the neck with your finger?

If i have understood correctly, yo have rattle between nut and 8th fret, when you play at this 8th fret, right?
if yes, you have to adjust your trussrod to give your neck some relief too.
the dead part of the string is vibrating, like a sympathetic string, so if it's to close to the fretboard, it can hurt the frets.

Have you tryed to replace the tuner?
Or the nut?
maybe the trussrod cover is a bit loosy?

What kind of headless tuner is it?
Is it a Daniel Fernandez bass?

you can try it.

Maybe the ground wire you soldered on the jack is not on the good lug, there is one for the negative from the battery and one wich is connected to the circuit ground.
when you plug the jack, you connect negative to ground.

ok, but if the bridge is wood, the strings won't be in contact with these screws, right?

Thank you

Je suis en franche comté
La mayenne, that's a long way off.

I think you have to shield the cavity, (copper is the best).
and if you have hum, maybe you can drag a ground wire from the tailpiece (is it body ferrules?) to the ground on the jack.

Hello all,
i'm Cyril, a french luthier.
what i prefer is building basses, headless mainly.
il like to "play" with bass design, and with woods of course.

the most important : my english is not realy good (not good at all ), so sorry for that, i will try to make correct sentences, but don't hesitate to correct me if i'm wrong.

see you in the forum