Run, don't walk, to your nearest Online bookstore, to buy F. Alton Everest, "Sound Studio Construction on a Budget". And look for his "Master Handbook of Acoustics" in libraries too. (10 years earlier than "Sound Studio", not nearly as good on current RPG-type devices, but extremely useful too, and it has a "model" room about the size of yours, with room nodes worked out). Your room IS big enough, suitable treated. Good luck--it's a fun project.
