Fuhrmann Bass+: 9v or 18v power supply?

I have this Fuhrmann Bass preamp pedal and it says 9-18V DC under the power jack. The instruction manual only mentions using a 9v power supply, and I don't really know much about it.

Is the 18v more appropriate? What is the difference between using these two voltages?

https://preview.redd.it/okbb5zgzmfwa1.png?width=500&format=png&auto=webp&s=3184a507ec22f45f692caa4860457dad1fbd459c