For lossless operation, you should set the application to 100%. Check with the Bittest … Only with a volume setting of 100% at the application, you can get a lossless signal path "end-to-end" (from application to the DSP of the ADI-2 Pro).

The final volume you adjust at the ADI-2 Pro.
Features like auto ref level support you, that the ADI-2 Pro will select the optimum reference level according to the volume settings to maximize SNR and Dynamic over a wider range.
If this should result into a too low volume setting at the ADI-2 Pro (maybe levels below -30 or -40) then you should think about using passive attenuators.
These are neutral to the sound and enable you to use the ADI at output levels more in the range of 0-30 …
See also this thread: https://forum.rme-audio.de/viewtopic.php?id=25399