In my experience, it's awfully tough to set a subwoofer's level by ear. If you're up to a bit of effort, get yourself a sound meter from your local Radio Shack and one of the set up disks that's available (I use "HD Basics", I think it is). These are really useful for getting your monitor set as accurately as possible without hiring a professional, and for setting the levels of the various possible speaker setups for a home theater. The disk walks you right through the whole process. Really simple, and it gets things quite close to just right.
