If it is a low frequency hum, it is most probably a ground loop somewhere, unless the problem is within the unit itself. I suppose you have to get a cheater plug and experiment with the powercords, taking care not getting electrocuted. See to it, that all your units are plugged into the same wall receptacle and experiment with a grounding wire between your OM 150 and your preamp. If all that doesn't change anything, try another pair of interconnects. (Sounds crazy, but for me that helped once)