Is the sub currenly on the floor, or on a shelf? A sub should be in a corner, on the floor. Moving it closer, or away from that corner (inches, not feet) will control the boominess. If you remove the sub too far from a corner, you'll lose quite a bit of bass. If you have your Logitech surround speakers hooked up to a PC, try using an eq to drop some of the midbass in your subwoofer (ie 100-500Hz range).