Polk PSW110..Not enough??!!

So, I'm just about finished my garage theater conversion..It's just a 5.1 set up right now...The sub is a Polk psw110...I was using this in my living room (12x16) and it would shake the walls..I now have it out in my dedicated theater (16x25)..I have it at about 75% up on the sub and +3 on my receiver...It sounds nothing like it did in my living room, even after turning it up...It just seems like a mid range and doesn't hit the lows as much..Is it just because of the size of the room, or do the acoustics play a bigger part??...Should I add another sub or just get one nicer one??
I figure I could sell the one I have for about $120..So I would have a total budget of $400 to spend on a new one..