I found some good information on trimming the N570 to help remove unwanted distortion. The chip contains to two identical sections which are mirrored on either side. Pin 3 and 14 provide an input for a control voltage for the gain cells. Pin 8 and 9 provide a distortion trim fro the gain cells.
Not sure exactly what is happening here. I suspect that trimming voltage of 3.6V is related to the internal bias of 1.8v used by the NE570.