I would start on front. If you are set up along the long dimension, then the rear wall will be less important. My rear wall is pretty far back and I have book shelves there. That has given the biggest gains. Then sidewalls. You can simulate with heavy blankets or carpets.
Front or Back wall diffusers
Front or back wall diffusers, I have a 14' x 24' x 8' stereo audio room with Wilson Sasha DAW speakers. I want to know which wall to place it on. I have seen many photos with diffusers on the front wall, this is the most recommended, but I have doubts. Please some suggestions.