+1Placement could be the problem.
Sit the sub in the primary listening position and crawl (yes crawl) around the room listening for when the sub sounds good.
Once you find the best spot, put the sub there and then listen from the main listening position.
Hopefully it will sound much better.
Corners often work well, particularly corners near the key seating(when the sub may be small for room size). Also, don't forget to re-tweak levels and phase each time you try a new spot.