Hi Ted,
First of all, that's a very nice, clean-looking room. I love the hardwood and the high ceilings.
Placing the M3s on either side of the doorways puts you at the sweet spot of an almost equilateral triangle 18 feet away. You can't get any better than that. Just make sure you toe them in a bit.
Your ceilings appear to be about 12 feet tall. You'll have a direct acoustic path length of 20 feet from each front and a reflected path length of about 28 feet from the ceiling. This will give you a reflection delay of about 7 msec and a reflection level of -3dB at your sweet spot. This is wonderful because that ceiling reflection will result in some image broadening when you are listening to stereo. Don't you dare treat that ceiling
. And ditto with the floor.
You also don't need to worry about any reflections from your side-walls. Given your room dimensions, the reflection level will be low enough and the reflected path lengths large enough so that direct and reflected wavefronts will almost be perceived as a single sound to your ears. You will experience some perception of spaciousness but not large enough to create an echo.
Another advantage with the reflected sound is a reduction in the amount of power that you will need. You will probably be fine with a couple of watts nominal.
Given the width of your room, you'll have a first mode at 28Hz. The height of your room will result in standing waves at 47hz and its multiples. And it looks like your room is 30 feet deep giving rise to modes at 19Hz multiples. At your sweet spot you'll likely be immune at the low end but will start experiencing cancellations above 100Hz.
You may want to think about corner treatments in the future and maybe some treatments at the front and back. I would discourage you from treating the sides though as that will add expense and possibly require a power increase for no good reason.
