Based on sound advice I bought 6 pieces of John Mansville, 2" x 2' x 4' with FSK facing at Steels. They do not stock Owens Corning with and backing and only have the 703 product and not the 705.
I put 1 piece in each of the corners and 1 each on the front and back walls. Although the frequency measurements are within 1 db of the untreated room the boomeyness has disappeared. The room seems to have been tamed and the midrange is clearer and more transparent.
It isn't perfect but I can live with it now. Thanks again for your help.