I keep my subwoofer in an inclosure on a shelf in the corner of the room. I wouldn't put it on a shelf too far above the ground, but it really doesn't matter where it gets placed. Having your subwoofer near the wall or in a corner can make it seem louder because the sound bounces around more. If it's in the center of the room then it usually needs to be turned up. If your subwoofer is an a place where you can't hear it well you can always turn it up so you can hear it, without losing any quality.