setup question with rekordbox dj

i have 3cdj 2000 nexus 2 and a djm 900 nexus 2. I have the 3 cdjs connected to a usb hub that is connected to my pc , the djm 900 nexus 2 is connected to my pc and used as a soundcard. Got all the settings correct on rekordbox and set the mixer to usb rather than line but when i do i get an interfering noise like a crackly noise coming through my speakers also records on my mixes, so something is interfering.So i tried a ground loop isolator on the line cable connected to my pc. did not help. Any ideas ??  

Also I tried setting the sound card to cdj Aggregator that works with the sound problem i do not get it but only 2 of the cjds work the other one is just a soundcard and does not come up in the option to set it ??