There are legitimate reasons why it might look like this... this is the sound that was flying through the air. If this one tape looks like this, I wouldn't get too excited. If they all look like that, then I would. It certainly doesn't look brickwalled at a fixed point. Since you are using using a semi-experimental mic/power setup, it's possible the mic body amplifier circuit is starved in such a way that doesn't fully put on a full signal linearly across the entire range.
Has your V3 ever had the "polarity fix" done? if not (and mine hasn't) the wave shown on the screen is actually upside down, and the "short side" is the top. It doesn't make much difference when listening, but this would be significant to someone trying to diagnose a starved circuit.