From the sound of things and what you already have, I would not spend money on a subwoofer until you can maximize what you have. Play a signal or song with plenty of bass, put you sub where you sit and move yourself around the room until you find a spot of spots where it sounds best. Those location(s) are where you sub should be.
If you can't find any good spots, then I wouldn't bother to spend money. Subwoofers get expensive quickly. In my case, I had a decent Velodyne sub that only sounded great in the worse parts of the room to watch TV. I tried to best it with EQs and DIY sonotube sand bass traps which helped, but it wasn't until I got a undercouch sub that was 1/5 the cost that went right under the viewing position where I eventually was BEYOND happy.