Thank you Mike. In the meantime, I have managed to let Roon play to a group defined in BlueOS, see this message. Rebooting one of the speakers and/or redefining the speaker group in BlueOS seems to have been necessary to enable Roon to detect the group, but in the end it worked and the stereo settings made in BlueOS are used when Roon plays to the Bluesound group. I am glad this works now. But still it is good to know there is a way to configure stereo settings in Roon too.
Greetings,
Frans