If your loudspeaker is more than five years old, you haven't heard what a modern tweeter is capable of. Unfortunately, like great cars, fine wines and beautiful women, great tweeters aren't cheap. The absolute worst shape for a loudspeaker would be a square. All surfaces would be parallel and equal in distance. Most loudspeakers are rectangles, which are only slightly better than squares.
Rectangles still have all parallel sides.
Furthermore, each seam or corner is a stress point. Building rectangles, there are two options. Either you brace the loudspeakers as much as possible or you stuff them with some dampening material to reduce the amount of energy trapped inside. Beyond driver selection, the next consideration is cabinet. There is much documentation that describes how a loudspeaker will act in a particular cabinet.