The piezo/magnetic blend is controlled by a knob that has a center d?tente position. Turn the knob all the way clockwise and there is magnetic signal only. Turn the knob all the way counter-clockwise to obtain piezo signal only. With the knob in the center detente position, there is an exact 50/50 blend of piezo and magnetic signal, and any other setting will be blended according to where the knob is set.
The piezo bridges are not available as a retrofit for earlier Music Man basses.