speaker placement in a tough room, please help.

I would recommend firing them from short to tall wall. The reason for this is 2 fold. One you will get a slight gain from going from a smaller area (volume), to a larger one. You effectively decreasing pressure through the room and will have the highest pressure behind the speakers. An added benefit to this is that by creating known pressure areas like this it is easy to acoustically treat them. The other reason I would fire to the high side, is that if you were firing to the low side you would get a reflection point off the ceiling behind you. It would be a long reflection time and would make the speakers less clear. You could treat the ceiling to get rid of the problem, but the easier solution is to just not work against nature and fire from short to long wall. Hope that helps.

I re-read your post and may have misunderstood the slope to your room. Is it centered about the room--so sloping in one direction from one side to the other? If it's a continuous sloping then I did read it correctly the first time, but if not my answer needs serious revision--please clarify.

Dave: Then I stick with my first response. This is an area that none of the software programs address. You can model things in both areas, but you will likely not get very different results other than some minor modifications in the RT-60 response curve (Reverberation times for 60 db attenuation). Software programs do not address the psycho acoustical nature of what occurs. We're considering do some significant research and development in this area, but it's very expensive.

Dave: This is not an uncommon occurance. The reason has to do with the reverberation times of a large vs small room. In a small room the reverberation times are short and they actually add to the spaciousness of the image because psychoacoustically we hear these short reverberation times as part of the original signal. In a large room the reverberation times can be too long and they are perceived as a seperate signal disconnected from the original signal. The result is image smearing. One other thing that happens in a small room is pressurization of lower bass notes. You need about 28 feet to have a half wavelength of 20Hz. At whatever your maxium dimension is (diagonal is accepted) this will accomodate the maximum 1/2 wavelength. Any frequency lower than this will pressurize the room. You can get a bass gain from this (this is how car audio works in low frequencies). It is not as natural sounding, but if it occurs where the speakers naturally roll off it can create improved bass response. In general, large rooms can work better, but they have to be properly attenuated so that reverberation times at anything less than a 60dB drop are kept to a minimum. The biggest problem I see with your room is that 2 dimensions are very close to being the same and the ceiling height crosses through both of these dimensions as well. You are likely to have a boost around 50-55 Hz. Low frequencies like this are difficult to deal with using passive devices and usually require active equalization. If you feel the bass is not correct I would suggest you measure the room acoustically and see what the response is.

As to the driver question it's really the area of the driver that determines how much air can be moved. This alone does not determine the roll off frequency, but two 6 inch drivers can move as much air as one 10 inch driver. In addition a 6 inch driver has less momentum than a 10 inch driver and thus is easier to control. The result is typically two 6 inch drivers will deliver cleaner sounding bass notes than a single 10 inch driver.