Thanks Massimo for your advice which I will definitely follow. Just for the sake of clarity, the decrease in bias to eliminate random crackling is not 2:3 = 0.6666 degrees but between 2 and 3 degrees. my fear is that the individual crackles could be caused by a loss of insulation. the panels were redone about 4 months ago in Germany by a company that previously produced electrostatic speakers but now dedicates itself to servicing planar speakers and not really due to an insulation problem.