M-Audio 2x2 hissing when monitor speakers connected

So i just recieved my jblk lsr305s and when plugged in they make the line to my headphones hiss. the speakers themselves dont hiss just through the headphone out. if i unplug them from the unit or even from the speaker end the hissing stops. also speakers on or off it doesnt make a difference it hisses anyway i dont want to have to unplug my speakers everytime i want to use my headphones i tried with a friends rokit 5s and the same thing happens so im sure its not the speakers :S any help would be appreciated
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