When you have a pair of speakers and a subwoofer, and you are trying to blend them, there are two options. One, where you play both the speakers and subwoofer full range, so the subwoofer is just "reinforcing" the bass. In this format you might get something that sounds good (if you tweak it right), but might measure weird. Alternatively, you might never blend it correctly.
Second, is to blend the system with a crossover. What crossovers do is put a gentle rolloff on subwoofers top and, and a speakers low end, like this curve (the three curves are the subwoofer, midrange, and tweeter curves, individually measured and then plotted together on a graph).
These curves add up to complex sum (as similar frequencies reinforce each other), so that the sum is a measured flat response, like so:
The easiest way to get a crossover network in your speakers and subwoofer is to simple get a receiver, which usually has a set rolloff for both speaker and subwoofer (-6db/octave) and a variable crossover, say 60-120hz. Generally you want to set this crossover as low as possible, to limit "boomy" upper bass, and anything above 80hz is highly localized. You can "hide" a sub better with a lower crossover because sub-80hz frequencies are perceived as unidirectional.
It's harder (and more expensive) if you want to truly "integrate" your Swans with a subwoofer, as they are powered speakers, whereas most receivers are set up to power passive speakers. Some receivers will provide line-outs for powered speakers, but if you do go the receiver route, you can buy better passive speakers than what the Swans provide for the money. With the Swans, you can certainly hook up a subwoofer easily with a cable splitter, but the subwoofer will just be "reinforcing" bass and the measured response won't be linear, and it might even give you peaks and nulls as same-frequencies might cancel each other out (destructive interferrence).
There's not much audio knowledge needed to set up a component system, just a chat with your local dealer or you can ask me what you'd like to know (I'm not a knowitall, as some people have accused me of being clueless on this forum, but I'll try to help where I can and you can take what I say and verify it if you wish). If you have any other questions, feel free to ask.