Room corners and placing speakers close to room boundaries; floors, walls, cause / result in the reinforcement of low frequencies that can smear detail in the midrange and low treble. It is necessary to remove this excess energy or absorb it through the use of bass traps.These are large (due to the long wavelengths of the frequencies involved) in order to be effective at low frequencies.
The sound you hear at the listening seat is a combination of direct energy from the speakers and reflected energy from the room boundaries. As frequencies increase, they become more directional and behave more and more like light, i.e. they reflect off flat, hard surfaces. If the reflected sound arrives at your ears within too short a period than the direct sound from the speakers, it will confuse your brain and result in loss of imaging and soundstaging. This is because the reflected sound must arrive at your ears later than approximately 7ms after the direct sound from your speakers so that your brain can distinguish direct from reflected sound. This is primarily a problem from sound reflected from what is called the first reflection point. That is the point along the ceiling or wall at which the direct speaker radiation first strikes and reflects off at an angle that will hit the listening position. The way to prevent this and to improve ambience in the room is to diffuse the sound striking off the floor. This is done with curved devices or irregularly shaped panels. These are best characterized by half round columns from companies such as ASC or square panels with irregular, skyline shaped surfaces from RPG. Their placement is determined by having someone sit at the listening position while an assistance moves a mirror down a wall adjacent to each speaker. The point along the wall at which the listener sees the speaker adjacent to it reflected in the mirror is the point at which the diffusing element is placed. The same can be done along the ceiling.
There's much more to this, but this should be a start.