I'm not faimilar with the MV100, you say you ran at the lowest level of -20dB, but just looking at the pdf manual it seems there are only 3 selectable gain values of +20/+40/+60dB. Is this correct, or is there an attenuation option as well? If so, a minimum gain of +20dB is a hell of a lot. I also understand Busman mics put out a hot signal anyway. So a hot mic signal combined with +20dB of gain might be something to do with the problem.