Hum is almost always the result of a ground loop—too many grounding points resulting in induced hum coupling through the amplifier. Systems are complex devices with numerous ground attachment points. A/C mains ground varies throughout many homes, various source components can have different grounding schemes and are all a potential source of noise. The best approach to tracking noise issues is to start by disconnecting everything from the REL except for the a/c mains power. If the hum goes away, then the REL is not the origin of the grounding issue, only the victim of a problem upstream in the system. If the hum persists after the a/c test then re-connect the High Level but lift the black ground wire. If the Hum goes away, turn on some music and if the REL operates normally (it usually will) then your problem is solved (ground has been obtained through some other connection point) and the black wire should be clipped and taped so as not to make inadvertent connection in future.
Another possible solution to eliminate hum is connecting the REL to chassis ground on the main amp and connecting REL black ground to an unused line level (RCA) connection. In some markets, especially North America, using an a/c mains ground-lifting plug on source components can eliminate hum, as this is the source of the ground loop. If all else fails, disconnect the high level and connect via the low level input as your system’s ground problems are beyond the scope of this FAQ section to resolve.