Klipsch sub12 not enough?

[BLindsay]

So my bedroom is 12x15, not a huge room by any means and i got a good deal on the klipsch sub12 but maybe its just me but it doesnt seem to fill my room very well ive tried moving it all around the room and its better now and i can move around and it doesnt really seem any stronger anywhere else in the room so i dont think its a posistioning issue. should i get a second one or should i just upgrade to something better? In my car which i like the way it sounds/how loud it is, is my Alpine type R on a 600wrms amp and in a car its brutal (140db) i was at a static-x concert the other night and my car reminds me of a concert with bass that you can feel hitting your chest but my home system is alot cleaner regarding the mids and highs but the low end seems lacking. How big do i have to go to really fill a room my size?