I have a new MD20 Stage scape and it works awesome except that our drummer uses 2 overhead condenser pencil mics with AA batteries instead of Phantom power. there is a low frequency hum/rumble in the speakers which disappears when we unplug these mics. It doesn't increase in volume as we wind up the overall volume but its audible enough to be annoying when we are setting up. it does it regardless of channels we use and on both mics so I'm sure both can't be faulty?