T3GGG,

I am sorry I do not clearly get you. Are you asking about your speakers or about room?  If you are talking about room, which the subject of your posts suggests then will it be a dedicated room or integrated room where your playback will be and what some living will take place. The reason I ask because you call your knew room as “great room”. You are look like from Australia but in US we have a fixed definition for “great room”.

http://en.wikipedia.org/wiki/Great_room

So, in the vocabulary that I accustom a “Dedicated Music Room” and “Great Room” are not the same.

I do feel that Dedicated Music Room and Integrated Living Rooms with high-end playback (Great Rooms) do require drastically different approaches to design. I can collaborate the subjects of Integrated Rooms, as it is something that I have been observing and thinking for years, not to mention something that I have recently built. I however am not a very good adviser about Dedicated listening Rooms. They are for sure are wonderful hi-fi playground but I never had any interest to have such a room.

T3GGG, I think the finishing of walls, ceilings and the rest things shall be less of your concern now. Sine you are not familiar with the horns you plan to build it might be a bit tricky to you to evaluate all pitfalls and opportunities that your given plane might have.  You need to start from the main 4 reference point

1)      Location of speakers

2)      Location of listening spot

3)      Location of equipment

4)      Location of bass horns

After you nail down all of it at your plan then, ONLY then, you might think about other things.

Try to visuals what you are trying to say via some sketching program as you said – it is very difficult to communicate otherwise.  I find that free Google Sketchup

http://sketchup.google.com/

is very useful and even fun to us.