Problem to control amp volume within Roon app using Cambridge control bus from network player to amp

I have a 851N network player and a CXA81 amp from Cambridge.
I’ve connected them using their RCA control bus in order to be able to control the volume on the CXA81 from apps.
It works when I use the StreamMagic app, but on Roon the volume control of the 851N audio zone is always marked as fixed. Does Roon takes the volume change possibility from 851N internal preamp user settings ? I’ve set this preamp setting to fixed on 851N in order to change the volume on the amp only when I change it from apps.
How can I manage to control the volume of my amp from Roon app ?

Did you ever figure this out?
I’ve just got the 851n and I have the same problem. Looks like I will be returning the 851n if its not fully roon ready.

You don’t control the volume of the amp, you control it via the 851 and its pre-amp function. For that you need to enable pre-amp mode in the 851’s settings. Then you need to enable in Roon under device settings 851 as volume control by device. That should work and I have it set that way with my Nubert amp A and the 851. Control link only partially works, turns amp off, but not back on.
. If not, the control bus link between the 851 and CX81 could mess it up.