Although all the discussion have been interesting, the original questions and limitations you put forward were that you couldn't afford to put out lots of money on a consultant and that you had restrictions on the room dimensions.
Rather than focusing on I'd specifics first I'd ask you to identify clearly, the limitations of the space (size, windows, doors), the actual usage of the room (just you, multi-channel), cosmetic restrictions and the construction costs. After those are clearly identified then work on the practical aspects of the design.
Q. Floating floor
A. Floating floors are most often used to isolate rather than improve the interior acoustics of the room. I prefer to seperate surfaces (walls, ceiling, floors) into two things, lossy or isolating. As Mikelavigne already said he has a concrete floor (concrete is not lossy but is generally isolating) with no floating floor and it sounds great. That probably means that the lossy part is in the construction of the walls, ceiling and acoustical materials (bass absorbers).
I prefer predictable spaces (flat ceiling, parallel walls, shoebox) for the basic structure constructed as a damped diaphramatic shell using sheetrock. There are several good ways to make a wall damped and lossy or damped and isolating (ASC Wall damp system as example). Low frequencies are predicitable. We know pretty close to where the speakers should be and where we should sit. You can always add bass absorbers that changes the shape of the room baised on the needs of the room.
Q. Ceiling Design?
A. As I stated above I like predictable lower frequency spaces but that doesn't mean we can't add more materials to the interior to change the shape and behavior of lower frequecies and up.
Clouds built into, on or suspended from the ceiling work great. The choice of materials is strictly dependent on the rest of the acoustical materials in the room. If you have adequate broad band absorption on the walss then the ceiling usually diffusive concentrated between you and the speakers and over your head. You can use RPG Skylines, Omni's or only the open finished cavities of the floor joists above. or, if you have now where else to put broad and absorption the ceiling is a great place to get it out of the way without usually doing much harm but much good.
Attic spaces with pitched ceilings are an excellect places to suspend a cloud over the pitch or directly attached to the ceiling.
Q. Diagonal rooms
A. Really hard to do well. You have the built in benifit of very low first reflections since the sidewalls fade away but now you have to figure where to sit. Most of the time they work well if you set up the room like a mixing room with the speakers as near field monitors with the rear walls a ways back. Those base modes want to react with the parallel walls even if you have the speakers outside a corner and the rear walls now have first reflections that wouldn't occur otherwise. So you are forever deciding which mode/reflection to treat and mode to avoid/not avoid. Have I seen any work, absolutely. Have I seen more fail, absolutely.
The room size is the room size. The axial modes take precident (LxWxH). The tangentials and oblique modes (longer wavelengths)are general down in strength 6-12dB and only could be benificial if there is a concurrance of a weak axial modes with strong "other" modes.
Please anyone chime in. I really like a discussion of the issues for a non-price no object individual. I'm always interested in technical reasonings and lesser so with anecdotal experiences but they still may be helpful.
Dbk, maybe you could clarify my ealier questions to help us (the forum) help you sharpen your focus.
