Panels to treat a vaulted ceiling?

My listening room is 16x23 with a vaulted ceiling, hard floors, hard walls and sometimes hard to listen to. I listen across the short side of the room using only 2/3 of the width leaving the highest part of the ceiling about a foot to the right of the right speaker. Book shelves and a large rug have tamed the floor and walls. All is well at low listening levels but how do I control the ring and echo that 400 square feet of ceiling produce at high listening levels. This is the only room that gives me a problem. I saw the spell check thread a while back and hope I have not misspelled ceiling a half a dozen times.
Thanks for any suggestions.