I have to agree, sounds like a ground loop issue. But there are different ways to solve this issue, and to pinpoint the exact trouble ground is a headache. Try using balance RCA cables, or using a ground loop isolator. Another thing to check is the actual power plug. Was there a ground pin on the plug before? Sometimes people cut them off in case of emergency when they don't have a socket with a ground input. Last thing to make sure is.... Are you using XLR input? Using RCA to a self powered speaker is not really recommended due to the lack of surface area for ground, and cable construction. XLR are on the other hand, usually has the wires separated with its own insulation. Borrow some XLR's and try it, if not just invest in a ground loop isolator. Let me know if this helps or not!