I assume you mean Outputs not inputs.. And there are many subs with speaker level (hi inputs) that you can just feed it off your main speaker cables along with your main speakers, however as mentioned you will need a crossover built into the plate amp powering the sub, most will have this feature, both features for that matter.
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.