I'll second Kr4's reply. Assuming that the room is OK and that the speakers have enough space to breathe, my next step would be to try and listen to the speakers in another room/system. Take them over to a friend's house and plug 'em in.
So first step, check the room and speaker set up, second step, check the speakers. Then report back if the problem still exist.
So first step, check the room and speaker set up, second step, check the speakers. Then report back if the problem still exist.