The best way to solve a ground problem is to work backwards. Disconnect the poweramp from the preamp is see if there is hum. If not, then the problem is upstream. If so then the ML's may need to have their plugs reversed. Then connect the preamp without any inputs physically connected. The inputs must be disconnected because their grounds are active even if they are not selected. When you finally get a hum, try a cheater and see if it helps. If it does not help, then trim the edges off of the bigger (ground) plug so that you will be able to reverse it.
Grounding problems-- Help!
I just bought a McCormack DNA 1. I am using the creek OBH-12 passive device as a preamp and my source is a Rega Planet. My speakers are ML areius i's. I hooked it all up and I'm getting a nice hum at 60hz. Please give me tips/ suggestion. I don't know anything about grounding problems!