Your placement sounds unusual. Usually in a rectangular room, the (box) speakers are about 2 feet from the back wall, with the listening position around mid-room, which would be around 10 feet away from the speakers. In order to get the optimum image from most speakers, an equilateral triangle is usually used. This would make the triangle 12 feet on each side. I would measure this. Then play with aiming the speakers in and out fo rhte best dispersion, sweetspot and image. Now the ceiling, being shorter at the front, will cause a horn-effect, which should not impact the image, but may cause some boominess in the bass.
The other thing to do is treat the sidewalls and backwall. Sit in the listening position and have another person put a hand-mirror against the side wall and move it until you can see the tweeter from the listening position. This is where you will need absorbing materials on the side walls. I use 4" thick Sonex myself. It can be wrapped in cloth to address the spouse factor. Wall hangings can work reasonably well too. As for the back wall, this is often better treated with a dispersive reflector. If you have vertical blinds, these can be half-opened. Heavy curtains work well too. There are several examples of different room treatments on the Audio FAQ of my website: