I recently purchased a factory refurbished Roland DB-700 which had very loud 60 Hz hum without any instrument connected. A Roland authorized tech serviced it, and now there is only slight hum (still with no bass connected; adding an instrument doesn't change the situation). I'm used to my GK MB150 and 1001RB being almost dead quiet; with the Roland, in a quiet room I can hear the hum 20 feet away. Is it possible that this is simply normal for this particular amp? I appreciate any input.