Within the Sonos product range, the “speakers” are a combined amplifier-and-speaker unit. Different “speaker” models have varying power outputs. But they are designed as a unit, so it’s unlikely that speakers will blow. 
 

Different speakers have different efficiency so some will seem louder than others. In addition, different environments have different acoustic characteristics. And an outdoor environment will usually need a higher volume compared to that for speakers in a room since the sound in a room is contained and reflected off walls and ceiling whereas an outdoor environment has no such containment.
 

All of this means it’s very difficult to say how to set volume levels for different speakers and environments. 

Thanks for the reply. I understand your point. However I think there is a lot more to it than what you describe. I’ve been running this combo for years incliding the deck speakers. Perhaps I have to live with it.  

Is your Port set to “Fixed” or “Variable”? When connected to a receiver “Fixed” is preferred.

The numbers when changing volume are just that: numbers. They are meant to give you an impression of what you are doing, but are not meant to be compared to the numbers other devices use, especially when yo7 are using devices from different manufacturers.

The only comparison that can be made to show if the Port’s volume is low is to compare it to another device connected to a line in on your receiver, say a CD-Player. Then you’d know, but part form setting the Port to Fixed there’s nothing to change on the Port.

I’m not familiar with your receiver, but YAMAHA usually provides the ability to individually adjust the sensitivity for each input.