spitfire: i'm an A/V  installer by trade, so lemme tell you about subs. They're made to reproduce lows that are basically non-directional. However, most rooms have a "sweet spot" for a sub (where it sounds beefy but not muddy); the best way to find it is to go get a $20 sound pressure meter from
Radio Shack, set up the sub on your chair, play something near the loudest you'd normally listen, then move around the part of the room where you'd like to put your sub, checking with the meter until you see more level.  The idea here is to find the spot where it sounds "present" without cranking the volume. There's a world of difference between loud, murky lows and lows you sorta feel in your gut. You can use your ear instead of a meter, but you get the idea. Once you find a spot, put  the sub there, and  you should notice good presence without a lot of  muddiness. If you need to tweak it more, if your sub has a crossover built in, dial the upper frequency limit down to near the lower limit of your main speakers (usually @ 30-40Hz for little PC speakers).  Most of the time, if you boost freqs in the 80-250Hz range, it starts to sound muddy, so keep it down where you feel rather than hear it. The corner rule, mentioned in other posts here, is true to some extent, but that might cause more 'woofing" than you want. Bottom line: Subs help, and they don't have to be pointed at you, but placement matters. Experiment.