I have added acoustical room treatments to several small listening rooms including my current one which was built from scratch as a dedicated listening room. And I have used many of the leading brands of absorbers, diffusers, bass traps and other acoustical devices including ASC, ASI, Echo Busters, GIK, RPG, and most recently ZR Acoustics. One thing that I have learned is that eliminating the reflections off of the wall behind the speakers is more important than treating the other wall/ceiling/floor surfaces, although you cannot have great sound without treating them all. So if I had your room I would cover at least half of the wall behind your speakers with a combination of ZR Acoustics Sample Rate and Hybrid panels (which have proven superior to all of the others I've tried in my room) before addressing the side walls, opposite wall, and ceiling. The closer you move your speakers to the wall behind them the more effective the ZR panels are (and the fewer you need). Not sure how that works with a rear firing tweeter. Never dealt with such high ceilings or such a vast expanse of glass before so rather than speculate I defer to someone who has worked with something similar. In your position I would contact Delta Design/ZR and ask for a virtual consult. Your room is similar in size to some of the larger film and recording studios the've done.
