Rectangular versus fancy design speakers?

Because the boxes are less expensive to build, more of the cost is spent on the insides, which to me are much more important to the endpoint—the sound. So in my experience, at a similar price point, the boxes often sound better. I agree with an earlier poster that ATC is a prime example of this.

Parallel walls create standing waves, internal resonance, and can cause some output from a back wave. Square corners on the baffle cause diffraction issues, and square corners with a lipped edge are worse. The cabinet is a major contributor to the overall performance of a speaker, yet the vast majority of boxes are still 6-sided profit boxes with square corners. Why? Because they’re easier to mass produce and cost less to make. Similar decisions are often made for crossover parts, wire, and connectors too. There are a lot of variables in play that also have a performance impact, including bracing, stuffing, lining of boxes, wall thickness, crossover design and frequency points, driver alignment, etc. The output from the box (vs just the signal from the drivers) is a big factor in the resulting sound.  

How much difference all that makes is also a variable, but non-resonant, non-parallel wall enclosures with well rounded, chamfered, or treated corners and baffles are a step in the right direction to reduce internal noise, diffraction and reflections from the baffle. If applied with great driver choice, great design, great crossover parts, and setup well in a great room on a great system they can definitely be worth the effort. Anything less is simply a compromise from ideal, but there are pros and cons with every choice, with cost often being one of the cons.

That’s the theoretical view, and doesn’t mean our subjective preferences won’t prefer certain box resonances within a speaker. Many feel some box resonance adds a warmth, as evidenced by a strong following of vintage box speakers, and others. What we like is always subjective.