Jimbojrjb,
You need to have Rives Audio analyze your room. They will draft a plan for your room. Most if not all of the recommendations can be done yourself.
I did Rives Level 2 and it completely transformed my room. I think this is a much better approach then just guessing were things should go. My room has corner traps (custom made. I paid someone to do this but I could have done it myself if I had the time) that are tuned to my room. I also have combinations of diffusors, absorbers, curved diffusors and a crazy wave thing on my ceiling. I don't know what else to call it.
My room is a dedicated listening room and therefore I did not have to hid my room treatments but this can be done and Rives will design accordingly.
Have Rives, or another professional, design a room treatment plan for you. You will feel like upgraded all of your components. How's that for most significant upgrade?