Every structure has a resonant frequency and harmonics as a function of dimensions coinciding with wavelengths. This can be a feature or a bug, depending on what your design goals are. Ever notice how cheap automobile subwoofer "boom tubes" thump out basically one frequency? Designed that way for a maximum of sound pressure for a minimum of amplification and components. There are always trade-offs between fidelity, bandwidth, power, directionality, and cost. Another example is when you hit one string on your instrument, and another string vibrates too, even though you didn't hit it. It's also the reason you want the neck-to-body joint on your bass to be as tight as possible -- and the logic behind neck-through designs -- to treat the entire assembly as one sonic entity.
In our amplifiers, we want to start with as even a frequency distribution as possible, then we tweak the hell out of out EQs to mould the tone we want. Starting from dead flat gives us users the greatest base from which to tweak.
Everything matters in speaker design: the way the driver is designed and manufactured, the way the cabinet is designed and manufactured and assembled, and ultimately the shape and materials of the room in which you are playing. There can be some interesting effects like a cabinet with some resonant freqs sounding neutral due to sloppy assembly allowing some of those rogue freqs to get absorbed in the slightly moving joints.
Along with the addition of foam, there is another mod you can perform on inexpensive cabinets: tighten up the joints between panels. The ultimate way to do this is during the manufacturing process via carefully crafted joints like box joints or dovetail joints but this has to be done at the time of manufacture. An effective way to remedy low-cost manufacturing techniques is to add reinforcing fillets to the joints. Here's how:
Hi-Res printable .pdf image:
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Going to have to play my old Ampeg for a couple of weeks, though 
Welcome to the Fender Rumble Club, you are member #487 
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