The HGS-18 is perfectly fine for HT especially in a room your size. Two subs are usually required in a much larger room. My guess is that your problem has to do with the placement of your sub. Two sub will give you more placement headaches. If your sub is close to a front or side wall, I would suggest that you experiment by moving it away from each wall one direction at a time. If the HGS as a phase dial, you should experiment with that if the sub ends up way out in front or behind the left/right.
Two subs, double the trouble. Worth it if you need them though but I don'e think you do. Good luck.
Two subs, double the trouble. Worth it if you need them though but I don'e think you do. Good luck.