Hello @Christoph_Eichenseer,
The Kii Three with Kii Control does not support USB volume control, so there is no way to change the device’s real volume using Roon’s built in controls.
Roon’s DSP Volume (Software Volume in the sMS-200 settings) uses a high quality 64bit precision attenuation with dithering to the device’s native USB bit rate. While it is technically inferior to the device’s built in volume control, you may find that it sounds very good!
-John