Problem solved. It just required a slight adjustment of trim pot "T7".
If anyone else needs to perform this adjustment, here's how:
-setup a basic patch with the filter open, the sub oscillator level at max and the 5th turned all the way down.
-flip the MB upside down
-remove all the screws on the bottom cover
-tilt the cover/keyboard assembly up out of the way but keep the ribbon cable connected
-Locate trim pot T7 (if the unit is upside down, it will be a sunken-in screw on the mid-left area of the circuit board, labelled "T7")
-get a tiny slot or phillips screwdriver that will fit it.
-plug in the unit, attach headphones
-hold down a note on the keyboard and turn T7 until you hear the overtones disappear.