Stephane,
Like some others may have mentioned, it would be extreamly benefitial to add some acoutic bass traps in your room. Self made traps are cheap and easy to make and will no doubt help your bass quality in your room.
According to Rives the optimum position for your seating position is 38% from the back wall of the total length of the room. (if your room is 10 ft long, your ears should be 3.8 feet from the back wall).
Here is a link of some pictures of a bass trap done with some OC703 Fiberglass.
-Robb.
