All your advice would be appreciated!
A room like that gets the same general advice I give everyone else for every other such room.
Here's the short version which will get you 99 percent of the way there. All rooms need:
* Broadband (not tuned) bass traps straddling as many corners as you can manage, including the wall-ceiling corners. More bass traps on the rear wall behind helps even further. You simply cannot have too much bass trapping. Real bass trapping, that is - thin foam and thin fiberglass don't work to a low enough frequency.
* Mid/high frequency absorption at the first reflection points on the side walls and ceiling.
* Some additional amount of mid/high absorption and/or diffusion on any large areas of bare parallel surfaces, such as opposing walls or the ceiling if the floor is reflective. Diffusion on the rear wall behind you is also useful in larger rooms.
For the complete story see my
Acoustics FAQ.
There's a lot of additional non-sales technical information on my company's
web site - articles, videos, test tones and other downloads, and much more. In particular, see this article:
How to set up a room
--Ethan