Realized a couple of other things I meant to add:
The higher the sub is crossed, the more it's location can be identified. The Bose goes up to 200 hz - and you need to use the crossover in the bass module (note that it is NOT called a subwoofer). Usually, the receiver crossover is somewhere (or even modifiable) between 80 and 120 hz. The THX value is 80hz.
And note how low the subs go. For HT, that last octave (20-40hz) is a lot of the "fun" you'll see people talking about in the action tracks. If you start reading, you'll see how much people have gotten enthused by more powerful subs that can go below 30 hz with authority (look for max SPL at a specified freq).
Now even as soon as you get into ported/enclosed or front firing/down firing subs, you start getting into debates here. But I think everything I gave you up to that point would be fairly unanimously agreed-upon.
Do note how many of the major mfr lower-cost subs are already tailing off before 30 hz. If *I* was doing a purely HT system, I'd get one of the fairly good $500 speaker sets that went down to my crossover, and get a "kicka$$/kickin'/whatever-your-adjective" (OK, I'm old and don't know how you'd describe it) sub - whether it's DIY or one of the online guys SVS, HSU, Adire, or other brand/model that you know can dig deep and with power.
 Note already the diff recommendations that occurred because he posted as I was writing this one - just the speakers vs. the speakers and the sub. My reason for sub recommendation was that you were talking little speakers already, and you're just not going to get very low with them. Personally, I did as Mr. Connel suggested because I *was* going with floorstanding mains. I just was not impressed by most subs I heard in my original price range - I preferred my main speakers. I waited and got one with extension to 20 hz when my budget allowed. You have to make those decisions yourselves. [/edit]