Are you connecting the Wiim Ultra to your amp using the RCA cables?, if so then have it at max output/volume and attenuate to the desired volume with your amp.
The dac chip in the Wiim Ultra will output best signal to noise at the full 2v output, there may be an option in the menu to output a line level, I’ll have a look at the specs/user manual
Had a look, I’d set it at 100 using “fixed volume output”,
Fixed volume output – Select to fix the volume at 100 on WiiM, so you can control the volume on your preferred device or music streaming app
Line out level – Select the preferred Vrms for your speaker or headphone