Hums and hiss in my pioneer S-DJ50X

I dont hear any hums and hiss when i connect my studio monitors to my iphone using a 3.5mm jack, but with my desktop i hear hums and hiss from my studio monitors. In order to get rid of these hums and hiss i had to disconnect the grounding wire(2 ohms) from the power mainboard. How do i get rid of these hums and hiss?? I am using Fii0 dac which connected to both the studio monitors via 3.5mm jack to dual xlr cable and the other end of the dac connects to the Pc via usb.