The setting on the Port is about audio out of the Port.  There isn’t any setting in Sonos that can tell your amp to send audio variable rather than a fixed volume.  In other words, you need to look at the setting on your Marantz.  Since you’re using the recorded out on the Marantz, I wouldn’t be surprised if what you’re looking for doesn’t exist.

Many Thanks for your reply Melvimbe. 
sorry it is Recorder out on the Amp and Line In on the Port. I wonder why there is an option on the app to have a Line out Variable as Fixed. So would I have to connect the. line out from the port to Line in at the  Recorder input on the Amp to enable the volume control?

I think I tried this last night but don’t think it worked. Will try again though. 

There are adapters that will allow PORT to be connected to the MARANTZ speaker outputs. This will allow Volume control using the MARANTZ remote. Obviously this would also adjust any speakers also connected to the speaker terminals.

Operationally this might not be much fun because any Rooms Grouped with Line-In will also be adjusted. In remote Grouped Rooms where the MARANTZ remote is ineffective the user could be locked out.

I suggest that you connect PORT as if it was a cassette deck.