That waveform looks very much like what I got at the Boston VH show back in October. This was with a Visivox preamp/BB, but the conclusion was much the same: insufficient voltage to the mics. I've since taken a multimeter to it, and indeed, the Visivox was only putting out around 4.25V, whereas an old Sound Professionals battery box (alone) was closer to 7.5V. (I just measured this off the other end of a 1/8" cable: positive to tip, negative to the ring. Whether it's entirely accurate, I don't know, but it confirmed what had been hypothesized, so...)