As volume via iPhone and Airplay works this should also work if you use Airplay from Roon. But in this scenario you are limited to 16/48 so no hires output to the Marantz.
My recommendation would be to use UPnP instead. Roon don’t support it but I offer rooUPnP 
With it activated on your Raspberry Pi you can stream Hires via UPnP. There is a seven day trial License you can get for some few dollars. This is a recurring license every seven days. If rooUPnP works for you please let me know. Please don’t forget to cancle the test subscription before getting a lifetime or annual license for it.
In my download area you find the rooExtend-Box manual and the Quick Installation Guide helping you to set up rooUPnP. You also find a list of tested devices in there. There is one Marantz device in the list so there is a good chance that this solution will work for you.
If you prefer a free UPnP streaming solution you can also try out this solution and install it on your MAC for example.
Best DrCWO