The more you learn about audio, the more you will use the calibration DVDs available. There is a wealth of audio nirvana waiting for those that learn to properly calibrate their audio system.
Not only setting the proper distances and dB levels for each speaker and subwoofer, but testing the entire frequency range response with an SPL meter and calibration DVD. Subwoofer placement has the largest audio impact on a system. Without a calibration DVD, you will never find the best place to position your subwoofer or other speakers.
All too often I see subwoofers stuffed into a corner and against the wall. You may gain a few dB in this manner, but it usually shows that no testing was done to truly determine the best place for the subwoofer. This usually causes frequency peaks that can obliterate the lower mid range of the entire system.
Lots of good articles on system setup and speaker placement in the Audio University section of this site if you wish to learn more.