adjusting system setting on avhx1500

i have the pioneer avhx1500 im trying to find out why under system settings why the rear speaker adjustment from "full to " subwoofer" isnt highlight..its visable but you cant change from full to subwoofer since not highlighted.does anyone know what i need to do to get it highlighted to change from full to sub?my car has factory amp as well as  have two  subs in trunk.i have the aftermarket subs into preouts 3 s/w and the rca from factory subs to outlets 2 rear..does this have anythingto od with why this isnt highlighted to adjust?im thinking since im using subs this needs to be set at sub not full...my trunk subs dont seem to be thumping like they should..

http://www.pioneerelectronics.com/StaticFiles/Manuals/Car/AVH-X4500BT_OwnersManual111912.pdf Page 39