Use a crossover for subs. Many subs have this built in, or if you go with a passive sub you can use a product like our Rives sub-PARC.
Hooking Up A Subwoofer
How does one integrate a sub in to a system that has no inputs for one? I have an Aragon 8008bb amp and a musical Fidelity A3cr pre amp. This might be of interest to me if I go for smaller monitor speakers. Thanks.