hum when hooking up sub to preamp

I have a Jolida Fusion Preamp and it's kind of noisy. It does have a musical full bodied sound which I like. I am hooked up to a Parasound A21 which has gain knobs. I have the gain knobs dialed back about a 3rd and this eliminates the hum to where you can only hear it from several inches away. When I hook up a sub from the XLR on the Jolida now there is hum coming from the main speakers again. I guess my question would be is there some kind of on way transmission xlr or rca cable where the noise would not travel back into the preamp? Would this even help. Any other suggestions to get rid of the hum here? 

Thanks