I have not ever thought of chimney as a bass trap. What an opportunity to tune it for your acoustic needs. I would be all over it. I have a coat closet in the corner of my room and it a pretty good place to utilize the volume within and I even put in a sort of louvered door to direct/reflect some sound. To some degree I wonder if the heating vents in the room could be any help at all. I have experimented with opening some windows in the room, but that is pretty major stuff sometimes, and not the effect I am looking for. Makes me wince when I see high dollar equipment standing in front of sliding glass doors in the listening room though.