Adding a sub(s) is one of the most challenging things to get the sound response you want. Ideally, what millercarbon suggests is the most sensible approach to this challenge. The approach to adding multiple subs (sometimes referred to as "swarm") is ideal.
However if you are not ready to get multiple subs you can look at the various wiring techniques mentioned here. Or, you can look into integrating a sub(s) by using an active crossover with bass management capability. Those crossovers can be from reasonable in price to very expensive. My solution was getting an Outlaw ICBM crossover. Original retail was $250. Don't believe it's still in production but can be found in the used market. A crossover of this type, with sub out connection, can be connected between the pre and power amps to perform bass management as well as adjusting crossover frequency to the mains. Not the supreme setup but can make integration of a sub much easier. That's the most important thing rather that how you wire up the sub.
However if you are not ready to get multiple subs you can look at the various wiring techniques mentioned here. Or, you can look into integrating a sub(s) by using an active crossover with bass management capability. Those crossovers can be from reasonable in price to very expensive. My solution was getting an Outlaw ICBM crossover. Original retail was $250. Don't believe it's still in production but can be found in the used market. A crossover of this type, with sub out connection, can be connected between the pre and power amps to perform bass management as well as adjusting crossover frequency to the mains. Not the supreme setup but can make integration of a sub much easier. That's the most important thing rather that how you wire up the sub.