Yes, we have exactly the same problem and we haven't solved it either. The microvox does seem to settle down once we get going but it needs resetting when we start up at the beginning of the evening no matter what we do in originally setting up the band PA.

It does seem to be connected to not sitting still, too! I wouldn't have thought melodeon players COULD sit still.

My other half mentions special melodeon microvoxes, which we don't actually use ourselves- they are, he assures me, shaped like long bars. Perhaps you have those already?