Although I am just trying my way without knowing much about the technicalities of the way these components work, I think I am on to something that might eventually enable one of you to fix the denonavr component so it works with newer Marantz recievers.
If you enter your recievers ip address in a browser like <IP_ADDRESS>:10080 you shold get access to a web interface of the setup menu and where you can also do some very basic operations like turn on and off and select input source.
I then opened the browser inspect tool in chrome and observed what kind of information was sent when I clicked around in the web app. Eventually I discovered the Network tab in the inspect tool and there the REST commands (I think it is?) that were sent.
So far I have identified how to turn the reciever on and off:
On:
GET http://<IP_ADDRESS>:10080/ajax/globals/set_config?type=4&data=<MainZone><Power>1</Power></MainZone>
Off:
GET http://<IP_ADDRESS>:10080/ajax/globals/set_config?type=4&data=<MainZone><Power>3</Power></MainZone>
So at least it should now be possible to create some REST sensors in HA for the basic stuff. I will try that now. Oh I guess that is not how this works. Better to create some API call nodes in node-red.