If you have the factory subwoofer (the one under the passenger seat) you may find, like me, that it is way too boomy in the mid-bass. The subwoofer seems to reproduce a lot of mid-bass that the door speakers are already pumping out, and this causes excess emphasis in the frequencies where they overlap. Plus, it is just way loud for my taste. This results in, for example, male vocals being annoyingly boomy.

A quickfix to reduce the volume of the subwoofer is to use the fader - the subwoofer is tied with the rear speaker volume and so fading the sound to the front simultaneously reduces subwoofer output.

I found that the subwoofer can actually be adjusted for cut-off frequency and volume. There are two small dials on the side of the unit - the same side that the connector plugs into. However, you can not access the screw-adjusted dials without removing the subwoofer.

Below are some drawings from the subwoofer installation manual to help you get it off:

Remove the plastic cover:

Remove bolt:

The subwoofer is stuck to the carpet with some adhesive. I managed to wiggle my fingers and slowly pry it up. You might have some luck with a spatula.

After I peeled off what I could, there was still some stubborn adhesive. I used orange cleaner and the rest came right off.

If you want to relocate the subwoofer to underneath the driver's seat, you will need to take off the bracket.

There are two small screws that are labeled. As expected, they are both set to the default 12 o'clock positions, which probably explains why I hated the sound so much.

SUBWOOFER ADJUSTMENT
Cut-off frequency knob: The cut-off determines the maximum frequency that the subwoofer will pump out. Turning the knob counter-clockwise will decrease the maximum frequency and decrease overlap with the door speakers. I disliked that the sub-woofer was producing too much mid-bass, so I turned it counter-clockwise to the 8 o'clock position.
Volume knob: Can you guess what the volume knob does? Turning it clockwise increases the volume. I set the volume to the 9 o'clock position to decrease the volume.

The sound now blends more seamlessly with the other speakers but is still a tiny bit bass-rich the way I like it.

REATTACHMENT:
Once you get the adjustment that you want, you may want to use the bracket to secure it back into position. However, I found that some velcro (the hook side, not the fuzzy loops side) on the subwoofer secures it to the floor just fine - the carpet is a perfect velcro loop imitation. I have driven my car enthusiastically for many miles now and it has not budged. Without the bracket, you have a lot more flexibility in locating the subwoofer away from rear passenger feet and to readjust the subwoofer whenever you want.

RELOCATION OPTION:
If you want, you can take this opportunity to relocate the subwoofer to the drivers side. This way, you can feel the bass in your seat rather than subjecting your passenger to the thumping.

You will also need to move the connector so that is hangs into the driver's side.

Here's a picture from the short-shifter thread that shows you how to get the console pieces off so that you can move the subwoofer connector to the other side:

http://www.scoobymods.com/mods/wrx/images/1c_consoleback.jpg

If you do relocate it to the driver's side, you will not be able to use the bracket, so use some hook-side velcro as I described above.