Should I use a rectangular or round in-wall/ceiling speaker?
Rectangular speakers are generally best for in-wall installations and round speakers are most often used in the ceiling. At one time, rectangular speakers tended to sound a little better than their round counterparts primarily because rectangular speakers had more room to include a better crossover (the electronics behind the moving parts that determine which components produce each of the frequencies). Around 1999, we, along with many other high-end manufacturers, began producing semi-circular circuit boards that contained comparable crossover parts and could still fit behind a round speaker. The result is that, for the most part, performance is no longer a factor when choosing between rectangular and round.
Aesthetics is usually the best determinant. Most designers agree that round looks best in the ceiling because people are used to seeing other round items in your ceiling, primarily recessed lighting, so the more inconspicuous than rectangular. It is also easier to install a round speaker in the ceiling because you don't have to be concerned about properly aligning the edges of the speaker with the surrounding walls when cutting the hole.
Also, in most cases, ceiling speakers are mounted into rooms where the primary purpose is "coverage" as opposed to rooms where the goal is to provide a soundstage for a specific listening location. Ceiling speakers will sound the same regardless on which side of the speaker you are standing or sitting because he tweeter is mounted concentrically to the woofer. All HTD in-wall and in-ceiling speakers include a pivoting tweeter that can help make the speaker more directional by aiming the highest frequencies toward the listening area. Lower frequencies spread out more quickly, making it difficult to ascertain the point from which they emanate.
In contrast, rectangular speakers have the tweeter mounted above the woofer (when positioned vertically) or toward the middle of the room (when positioned horizontally). When mounted in a wall, it is easier to achieve a nice stereo image with rectangular speakers than it is with round speakers when the listening position exists directly between the speakers.
So if your room is laid out with a specific listening area, all other considerations being equal, rectangular in-wall speakers are preferred. If the goal instead is to evenly cover an entire room with music, a pair (or two) of round in-ceiling speakers will do the best job.