Joeb,
You might consider opening the ceiling to the roof (cathedral). I have a second floor listening room over the garage that is 18'W x 28'D with 9' walls and a 13' peak. It is very close to the golden ratio. The extra height really opens up the sound stage vertically. I used the Cardas formulas for speaker placement from the side walls, but kept the speakers at 5' from the rear wall. The image is phenominally wide, deep and high and forward of the speakers. This room is one of the best I have heard.
You might consider opening the ceiling to the roof (cathedral). I have a second floor listening room over the garage that is 18'W x 28'D with 9' walls and a 13' peak. It is very close to the golden ratio. The extra height really opens up the sound stage vertically. I used the Cardas formulas for speaker placement from the side walls, but kept the speakers at 5' from the rear wall. The image is phenominally wide, deep and high and forward of the speakers. This room is one of the best I have heard.