Yes, the sub "sees" the entire space, not just the room so if you have open spaces it won't properly pressurize the area. You might try a near field placement, meaning putting the sub close to your seat. This is what I did and got good results. Ordinarily sub placement is critical and near you might not the best for flat response but at least you will feel it.
You might also try a tactile transducer or bass shaker attacted to your chair, sofa, something like a Butt-Kicker:
http://www.thebuttkicker.com/