room shape

Spatialking, Good point WRT to solid vs. less solid walls. Obviously adobe bricks are not common in the US and there isn't too much I could find on their acoustic qualities, but they do absorb sound. Also since walls constructed this way are uneven, I'm sure they would diffuse the sound to a certain extent. Sheet rock with offset 4x4 studs is a possibility I'll keep in mind.
I've spent many hours listening near field in studio control rooms, but this is my first opportunity for a large room. My experiences with 5 sided rooms in that environment have been positive, but they have been small. Finding a specialist in the 3rd world could be problematic, but I plan to investigate that avenue.

BTW, one thing I forgot to mention is in a larger room you need speakers that can comfortably fill it with sound.  That is, they need to play more loudly with the same clarity and definition that smaller speakers would play in a smaller room.  

I am going to guess that most of us probably have speakers that should be in larger rooms than we have.  I am certainly guilty of that.   If that is correct, then perhaps this is not as much as an issue.

Though I haven't had any connection with Michael Green, I think he would be someone to contact, given his CV.
Bob