I had the exact same problem, and I too thought the only way to get rid of the hum was to disconnect the audio between my DVD (Sony) and preamp (Rogue). I tried another preamp and had the same difficulty, and another DVD as well. Finally I narrowed the problem down to the power cord of my amplifier. By moving it around the hum would get louder, softer, and in a certain position against the wall it went away entirely, so I taped it there. No more hum. I hope this helps.
Mike