You might try using a cheater plug to "float" the ground at the AC plug(s) of both components; I'd also try leaving everything as is (with the RCAs connected per usual) and connect a wire between both units to effect a chassis ground. There is NO simple solution for ground loops, or differences in ground potential. Trial and error are the only sure curatives. Most can be solved with patience; some cannot. Happy hunting.
-Richard
-Richard