Foam bass traps and diffusor panels

I agree with trying Rives formula and buying his Radio Shack corrected CD. Place your DB meter on a tripod at your listening position. Write down your db output at each frequency from about 32hz-200. Keep track with each speaker movement. Based on your positioning, I would guess that you would want your speakers closer to the back wall. Notice which frequences change, what direction, etc. Move in small incriments. Distance from side wall will also change bass characteristics at the listening position. In my room (Approx. 20x28x8.5, non-parallel walls and ceiling) my speakers are about 11 feet apart and 14 feet from the listening position. They are closer to side and rear walls than yours. I like the less "near field" quality of greater seat distance (mid-hall). As a general rule in my room moving speakers farther from the rear wall increased 60hz, and decreaased 50hz. I found a nice balance by watching the meter as I moved speaker position foreward/backward. I also found reduced 60hz as I moved the speakers closer together. Additionally, listner position can change any of these frequency measurements by huge amounts as you move a matter of inches. Rives is great to work with and you might want to consider using his bass EQ. He is designing a rooom for my brother now and he really seems to know his stuff.