I'll save the smart comments about speakers with better bass. I have C4's that are supposedly "superb" at bass. No, they are good at bass but need two DD10+ subs to be flat to 20Hz. Yes, it makes a difference. I have NEVER heard a speaker system sound effortlessly flat to 20 Hz without powered subs somewhere. No, not even FOCAL Utopia's, that are HUGE! Liasten to any Vandersteen Quatro or more expensive speaker simply walk away with excellent deep bass response. Why? Properly integrated powered subs!
How to hook up a sub, and were to put it ar two different issues. You try to put it where it is most linear where you are using it, and that denpends on how you hook it up.
Method one is to use the high-pass system that uses your main speakers down to their natural -3 dB point in the bass, or as close to that as your X-over choices are. You then splice the subs in at a point where they are blended in and extend down to what ever your sub can do. This makes the sub cover the widest frequency range so where you put it is more critical, but cuts the amplifiers load significantly on you main speakers. This reduces distortion a LOT as the mains woofers aren't moving near as much playing the midrange information, which really wants less cone excursions occuring during play (called intermodulation distortion). The draw back is "blending" the subs sound to your main speakers as they play "up" into the lower mids, and getting the sub linear enough over a wide frequency range to be flat enough (corners don't work so well).
Method 2 is to play your mains full range. Then EQ the sub to simply (not so simple, really)patch-in the response curve so it is flat. Here, the sub is being used less, but the mains more. This is easier to do as the sub is working mostly way low, so it's "sound" isn't as hard to integrate but it's speed maybe, though. Sometimes a simple wall or corner placement works fine, though.
The choice is either; how good does your main speaker do bass and maintain a clear midrange? If you feel this is the case, try a full range approach. Or, if you feel the speakes bass is less pleasing so use the high-pass option (especially if you are taxing a somewhat small mains amplifier).
You may find that getting the sub flat over a wider response range with the high-pass option is not capable based on your room (sub wants to be in the middle of the room!), forcing the full range mains and blending method.
Trust me, you will know it when you hear it. Try both methods and pick the one where you think that the sub isn't working! That's closest to right for you room / speakers.
