My opinion would be to cover the first reflection points with some 2" OC 703 treatments. Here are some pics to help show you where the reflections would be coming from:
The simplest trick I've heard is to have someone sit at where you watch movies/listen to music, then another person will run a mirror along the walls. As soon as you see the tweeter, you mark the spot and put the treatment centered on the mark you made. Your ceiling may be a little more complicated since it's not a flat service -- I might put a little extra there because of that.
Finally, you probably should have some bass traps in the corners.
The one thing you may need to be careful of is not to overdo it. Only about 35% of the walls should have treatments. I've been in an overtreated room. It had GREAT equipment. It sounded HORRIBLE because it was overtreated.
Then for some exrta credit, I'd read
this and
this. It's where I "borrowed" the pictures above from.
JCD