Sorry, Rick - John wins this one.
There were HEAVY echos in there when all I had was the blank room. I had added caulked 3/4" plywood and a layer of sheet rock to the walls, so they had a ton of mass - great for stopping sound in/out, but echoes all over the place inside.
The echos were greatly reduced as I put the sound treatment inside the room.
There *were* some last echos in there when I had the heavy solid door right across from the blank heavy wall. Once I put in the "diffusor" (bookcase), that took care of the last of the echos. I'll still add 703 to the door facing, but its pretty darn good in there now.
The key is that the slats in the slot absorbers are angled. To be fair, its hard to see that in these pictures, but each slat is canted at about 7 degrees - about 14 degrees total effective splaying for the walls. I don't have the exact figures here, but each slat goes from about 3" deep up to about 6.75" deep over the 2' span.
Last night I hooked up enough stuff to actually play music in the room. I am VERY VERY happy - my monitors have never sounded so good.
I want to say thanks again to all you guys -- I could never have done this without your help and knowledge. This place, and John's forums ROCK!
-lee-