Think of it in these terms:
On an acoustic guitar, what produces the sound? The vibration of the strings, right? So technically, any guitar with identical strings should sound the same, right? Wrong. The strings resonate over a cavity that amplifies the sound (the guitar body). How it is shaped and what it is made can alter its tone dramatically. Just listen the huge difference between a dreadnaught, a jumbo, and an auditorium size steel string. Tonewoods are also huge factors: cedar tops are warmer, spruce tops brighter. Rosewood, mahgonay, maple, walnut; all huge factors.
Now apply that to a speaker cabinet. Although sound coming out of two speakers may be exactly the same, the cabinet they are mounted in, and the way they are mounted plays a role in shaping what you actually hear. Different shapes, sizes, and materials will alter how a cabinet resonates. Different baffles and port configurations too.
Good cabinet construnction is one of the things that really seperates low/mid and high end amplifiers. Many lower end models use chipboard and low end woods for their cabinets, while highenders use premium-grade plywood with no voids.
Generally speaking, a bigger cavity will resonate at a lower frequency. If someone is telling you straight cabs sound better than slanted ones, it's probably for that reason. A straight cab has more room to resonate inside it that a slanted one, just as a full size dreadnaught
acoustic guitar has more room than an identical model with a cutaway, and produces a deeper and (many argue) better sound.
Aaron
http://www.aaroncheney.com