TBH I actually cannot remember for sure now. I have had to send an old and difficult to source remote (with volume control) to Krell for repair. This has all coincided with the new build and was the reason I noticed the issue with device volume in the first place. Because I had no remote I was trying to control the HQPlayer volume from roon.
The functionality in Fixed or Device mode is identical in the sense that there is no roon control over the device volume. All that is different in each mode is that in Fixed mode I do not see the volume settings in roon (as set in HQPlayer) but in Device mode I do see these HQPlayer volume settings in roon, it is just they are inoperable, i.e. I cannot adjust the device volume from roon.