Hi,
When you clap your hands in the room, is there an echo?
Your bare walls are an issue. I would take inexpensive towels, and layer them in a frame and staple them to the frame. Cover the frame with something attractive, art, or just a fabric that looks nice, etc, staple that down too. Hang these over your first reflection points. 2 inches thick should do it. For behind your speakers, do the same thing, but with 4 inch thick worth of towels.
Very best,