Keep in mind the size of sound waves: 13,200 inches/Hz. So 13,200 Hz are 1 inch long, 1,320 Hz are 10 inches long, 60 Hz are 220 inches long, etc.
If possible best to move the loudspeakers out 3ft from side walls and 5 feet from front wall. Try quality absorption panels if that's not feasible. By quality I mean to research the material absorption characteristics. Most "absorption" material is a sick joke. Even the "good" stuff isn't effective in the bass range. Again Owens Corning 703 high density fiberglass is the king. Accept no substitutes.