First, turn the amplifier on first. Mine hums if I turn the REL on before the amplifier. It did not hum with with my old amp off, but it does on my new one if the amp is off and the REL in on. May not be grounded with the amp off (the black wire). My current amp must lose the ground through the speaker terminals when it is off. See if that is the case. So I always turn the REL on last and off first.
Secondly, the Neutrik should lock in place. You put it in and turn it clockwise until it locks.