Have you invested in a copy of Robert Harley's book, "The Complete Guide to High End Audio"?. Many of your questions about room design, diffusers, bass traps, speaker placement, etc are explained in this excellent work. It's a book I refer to on a regular basis. Chapter 4 "How to Get the Best Sound from your Room" covers much of what you're asking about. IMO, it's a must read and re-read for all those in this great hobby. Available at major bookstores or on-line (Acapella Publishing).
One recommendation that my hi-fi store gurus tell me is to install wall bookcases in your listening room. These will help to remove echoes and turn down the brightness just right. Thick carpet is another important material to have.
One recommendation that my hi-fi store gurus tell me is to install wall bookcases in your listening room. These will help to remove echoes and turn down the brightness just right. Thick carpet is another important material to have.