Question

Listening in different volumes during mastering is essential.

Infact sometimes I don't wanna hear anything and just watch the meters or 
turn the volume all the way down, just to check if I can still hear everything
and the vocals still sit at the right spot.

Yes, you can turn down your headphones or speakers. But: 

Having a knob to change or mute the Listen-Volume in the project-page
without changing the faders, would come in extremely handy!