I'd read F. Alton Everest, "Sound Studio Construction on a Budget" before I did anything. Among RPG products, Skyline diffusors are relatively affordable and very effective on back wall (behind listener) or front wall. (Side walls I haven't tried.) Earlier advice sounds good to me, but for a comprehensive grasp of your situation, do read Everest.
Acoustic Treatments
I need some opinions on the above, my listening room is a partially finished basement with floor joists for a ceiling but the front wall behind the speakers is finished. One side wall is framed out and the other is cement with insulation (foil side out). The side walls are 5 feet from the speakers. The system sounds pretty good in this room (I've had the same in 2 other rooms). I noticed a big improvement when I hang damping material on the front wall between the speaks like a wool blanket or oriental rug. Can I improve on that with RPG or Sonex or the like (I know it will look better). I know tuning the room is critical, I've just never had much luck or inclination to do so. Any thoughts would be greatly appreciated!