If you're using the sub for a home theatre and want the hosse to shake when the bombs are blasting, that's one thing. However, if you want to supplement your speaker for music, then you shouldn't really notice the sub. If you do, then you probably have the crossover or volume set too high.
I would suggest that a sub should do two things. One is to extend the bass a little deeper than your speakers are presently capable of. As a result, a sub is often good with a small monitor in situations where the monitor doesn't go as deep as you would like.
The second thing a sub can do is add headroom. In other words, the sub can supplement frequencies the main speaker is already producing. Smaller sopeakers can usually go down fairly deep, it's just that the volume is too low. For example, a speaker spec may say 50 Hz, plus or minus 3 dB. This doesn't mean it can't reproduce lower than 50 Hz. It just means that when you go below 50 Hz, you will be more than 3 dB down. So the apparent volume of the speaker at frequencies below 50Hz will be so low that it isn't rteally noticeable. A sub can add the extra volume to frequencies the main speaker is already producing. This is a slightly different thing than just adding lower frequencies that the main speaker can't reproduce at all. This is one reason why you might use a sub with a floorstanding speaker that already goes down farily low. This is why some subs are set up to run in tandem with the main speakers, rather than relieving the main speaker's bass driver totally of the low frequency signal and sending it to the sub.
And in either application of course, there are potential integration issues.
So, as the posters above suggest, a sub isn't necessarily just to add more "powerful" bass, as it might be for home theatre applications, but to "supplement" your existing bass, either by adding bass frequencies you don't have, or by supplementing existing bass already produced by the main speaker. Supplementing doesn't necessarily mean more powerful. It means adding headroom to add to dynamics, although there is certainly the option of making it more powerful by increasing its crossover frequency or volume relative to the main speaker if that's what you want
If you're carefull in how you integrate the sub, it can add wonders to your main speakers without you even knowing that its there. I would rather have a main speaker with good bass response, rather than a speaker/sub combination. However, it's usually less expensive to go the main/sub route than it is to find a truly good main speaker with very good deep bass response at a reasonable price.
