I would also upgrdad the 859 to the DI Wide transducer Humminbird | XTM 9 WIDE DI 20 T
It is designed for the trolling motor and has the 83/200 2D which will give you better 2D coverage area.
If you add the wide DI to the 859 will the 83khz even work since the unit only drives the standard transducer at 200/455khz 2d? Is there a transducer selection on that unit that includes the 83khz?