Great stuff John and glad to help. The EP500 is truly one of the finest subs around but like all equipment it needs careful installation and adjustment to perform. Given the size of your room you were likely getting huge boundary reinforcement in the 20-30hz zone which was causing the rattling and making the rest of the band sound quiet in comparison.
One of the nicest features of that sub is the trim control which boosts output above 33hz. As you get used to the sub it may be worth going back and forth with the full and half setting. A full setting may give you a more level response but its hard to say without graphing the room with test tones, a metre and pencil and paper.
