High level sub inputs are there only because the majority of customers do not have the abilty to run low level inputs and do not want to afford an appropriate cross over. They are the poor man's way of running subwoofers. To do it correctly your preamp should be connected to a two way crossover which then runs to amps and subwoofers.
@emiliop , you can get a "y" connector to "mono" your signal for the sub.