First read F. Alton Everest's "Sound Studio Construction on a Budget", which deals with listening rooms too and contains many DIY ideas. You need to understand things like resonant modes (and calculate them for your room), reflections, decay time, and so on. In my experience (with a room roughly like yours), it took a long time to feel that I'd "arrived": you can put in crucial bits of room treatment that don't show their full value until ALL the needed bits are in place. If you have a table saw and are really a DIY type, I have free (except for $1 for postage and printing if you want hard copy) instructions for DIY RPG-style quadratic and primitive root diffusors (read about them in Everest) and Argent Room Lens imitations, which I'd be glad to send. Email me at email@example.com if you're interested. Everest also has a 4th edition of "Master Handbook of Acoustics", which should be very useful--but I haven't seen it myself.
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