We know that our rooms play an important role in sound quality. A 2-channel system that performs to spectacular levels in one room, might sound miserable in another.
We're asked all the time whether or not adding absorption or diffusion to a room is the best option. The answers are rarely clear and almost never easy because every room is different.
However, there are some general guidelines we can use to help our decision making.
The simplest is when we start with an obvious problem like slap echo or over reverberance. Clap your hands near the listening position and see if the clap sounds natural—without anything added or subtracted—or is there a quick echo slapping off a wall? Always, we're looking for natural sound in our room. Not overly dull and absorbed, nor overly bright or echo-laden. If you hear an echo that's likely a good indicator you need some type of absorption to kill the bounce at those frequencies. Corners, the meeting point between ceiling and wall, and large open wall areas are prime candidates for absorption. Use only enough to kill the echo without robbing the room of life.
Step two in room treatment happens after we've tamed the echo or overly live room with modest amounts of absorption.
Diffusion's tricky. While it's my favorite for improving imaging and smoothing tonal balance, it's easy to get carried away and wind up with a mashed potato soundstage—wide and deep but nothing has specific placement and individual instruments and voices sound diffuse.
Start with the point of first reflection using the old mirror trick. Employ a friend or spouse to stand with their backs against the sidewall between the speaker and your listening position. Holding a small mirror in front of them—facing the opposite wall, have them move closer and further from the speaker until you can see the tweeter from your listening position. That's the point where diffusion typically works best (for starters).
Diffusers also work well on the front wall behind your speakers.
My best advice is to go easy. Most people wind up overdoing the room and then regretting it.
Hope these tips help.