I think the problem you are dealing with is a simple ground loop. If you speakers or your audio interface have balanced inputs, connect the center of the RCA plugs to + and the ground to - using the two wires in a shielded cable. Connect the shield to ground on the balanced side, and leave the shield unconnected on the RCA side.
Please confirm that you speakers a active (powered) speakers.
 
If you don't have balanced inputs, try this:
http://www.guitarcenter.c...;kwid=productads-plaid^32411604521-sku^103815098@ADL4gc-adType^PLA-device^c-adid^18055319321
or
http://www.sweetwater.com/store/detail/SDirectPlus/
 
Wilko