When you use AirPlay, as you know, there are volume controls within iTunes that control individual zone levels and a master overall level. If that works for you, you don't need any additional controls between the amp and speakers. Those would be a convenience for anyone who is in a zone and finds the volume wrong, they can just reach for a knob on the wall and change it. If you do use those physical in-room volume controls, you'll want to run the iTunes AirPlay zone volumes all the way up, start with all your in-room controls all the way up, then adjust the iTunes master volume control for the loudest level you'd want in any zone. Then use the in-room controls to tune the volume down.
In-room volume controls between amp and speakers can only turn volume down, not up. They are passive and add loss to control volume, they don't amplify. That's the reason for the procedure I suggest.