Most subs have two inputs and two outputs. If it is a newer sub, it will have an LFE input which bypasses the crossover.That would be used for the main system, the other input/output could be used for the small amp. In my system I am using the LFE input for the home theater. The other input is used for my McIntosh integrated amp for two-channel listening and uses the crossover in the sub. If you have only one sub, since bass is nondirectional and in pre/pros the bass is summed, use of a Y connector is generally the method used.