18v doesn’t give you more output, it gives you more headroom before you clip the preamp. Overall volume doesn’t change.
Roger has said many times that it’s not a concern on his preamp and if I recall, he couldn’t tell a difference between 9v and 18v.
Have you ever added another battery to a Sadowsky preamp? I have done it 4 times and there is definitely more output/volume/headroom, I know it from experience. I currently own 3 Sadowsky basses (had 5 at one point) and they all have different output
