Apple Logic Pro /LogicExpress Discussion

Hi, 
I recently got my Motu Ultralite mk3, and have a little question if any of you could help me..
When I plug my Headphones I still can hear my main monitors (which are self-powered thus I 
don't use an amplifier)  .. I know it's obvious 
But.. is it possible to mute them when I use my Headphones? Otherwise I have to switch them 
off every time which is inconvenient and silly.

When I turn master volume down, it turns phones volume with it as well..

Thank you so much