Sixty-five head and Preamp juggling

any thoughs of what i could do so i could switch between the head being a power amp for the preamp and being a head, i thought of using a A/B switch box 1 going to the preamp and when i want to use the head just turn to a patch in the preamp that is 0 gain so there is no output and switching to the head, but im not sure, i think i may need to wire another input for either the input for the preamp, or an input from the A/B box?
thanks!!