Per F. Alton Everest's book (Sound Studio Construction on a Budget), there are three recommended ratios of room dimensions: 1) width=1.14 times height and length=1.39 times height, 2) width=1.28 times height and length=1.54 times height, and 3) width=1.6 times height and length=2.33 times height. Alternatively, the "golden ratio" rule is width=1.618 times height and length=2.618 times height. This assumes that height is the smallest dimension -- as long as these ratios are maintained, things are fine so if width is the smallest distance, calculate the height and length ratios based on the width (e.g., height=1.14 times width and length=1.39 times width).
Without getting into all the details, distances between parallel surfaces tend to reinforce certain frequencies (those frequencies which fit neatly into that distance). One likes to avoid too much of a gap between these frequencies and too little gap between these frequencies. A cube is an awful shape since the same frequencies are reinforced by each parallel surface since the distance is the same. Direct multiples (e.g., an 8x12x16 room -- all multiples of 4) are usually also a bad idea for the same general reason.
My rule of thumb is to keep the length of the room as long as possible since the longer the room, the lower the bass frequency that can be accommodated. An 18.33 foot room can handle a 30 Hz wave and up without the introduction of acoustic problems of one kind or another.
If you have an 8 foot ceiling, an approximately 13 foot width and 18.33 foot length works out pretty well. There will be a coloration (gap in response or weaker perception of the signal) between 90 and 123 Hz, between 185 and 212 Hz and between 217 and 246 Hz. The 217 Hz signal will seem a bit colored in the opposite direction (reinforced frequency or stronger perception of the signal). This is a pretty darned good result since there is no room dimension that will be perfect in all respects.
If you have a 9 foot ceiling, you can move more to the 13.5 to 14 foot range in width keeping the 18.33 foot length. You'll notice a slight peak in sound at about 61 Hz, 125 Hz and 251 Hz. You'll notice a dip in sound between 92 and 123 Hz, between 125 and 154 Hz and between 215 and 246 Hz. This is also pretty darned good.
All kinds of other factors also influence your overall listening experience from an acoustic point of view: The materials you use for the room surfaces, the position of the speakers relative to the listening spot and the use of absorption/diffusion materials are all important. Experiment with different materials and/or talk to an acoustic engineer/expert for help. Picking up the Everest books is also handy. The one I referenced is a reasonably easy read. Everest also wrote the Master Handbook of Acoustics (as referenced by Holzhauer above), which is a tougher book to get through.
Starting with the right dimensions is important. Acoustic drop ceilings may not provide you as good a result as a more solid ceiling. Non-parallel walls can also help since that helps to dissipate the reinforcement phenomenon. Unfortunately, I don't know enough to calculate the optimums here. I suspect that the walls are still relatively parallel in your sweet spot (sort of a calculus slope thing) so you may wish to apply the recommended ratios at that point. Acoustics is very complex and I don't want to get in over my head, so the advice about an expert is good if you think it's worth the bucks. On a rough basis though, the aforementioned ratios should work reasonably well. Good luck.
I invite any acoustic engineers or gurus to add their expertise to this.
