The square dimensions aren't doing you any favors at all, they are notoriously tough to work with particularly with a single sub.
My office is a bit more forgiving, but similarly small in area. I went from a somewhat larger single sub to a pair of slightly more modest subs (still plenty enough for the small room) and got better results.
Maybe return the pb2000 for a pair of pb1000? Small rooms get a lot of cabin gain, so sealed subs with their shallower roll off are an option without sacrificing extension. So if space is limited, a pair of sb1000 is a viable alternative.
Probably not the feedback you wanted to hear. Sorry, man. But square rooms are not ideal.