A speaker - actually everything - has a point where it resonates. An impedance curve shows this clearly where the natural resonant point of a speaker occurs. As the frequency goes down, at some point the electrical impedance rises sharply. It at this impedance peak that the resonance occurs. Measuring the impedance of a speaker in free air (not inside an box or enclosure) reveals its natural resonant frequency.
Speakers produce sound uniformly at frequencies above this resonant frequency (here, to the right or at frequencies above the peak). If the resonant frequency of a speaker is say 35Hz, either you must operate the speaker above that point or do something to change the resonant peak to get the speaker to produce notes lower than 35Hz. Stuffing the speaker inside a simple six-sided box (such as with acoustic suspension speakers) only raises the resonant frequency meaning that it will never produce notes down to 35Hz. Something else must be done.
Raising the mass of the cone lowers the resonant frequency (shifts it to the left - a good thing for deep bass) but the added mass makes the sound slower to respond to electrical signals and the resulting sound is muddier.
So what is one to do? Changing the kind of box you put the speaker into is another option. A properly designed bass reflex enclosure is tuned to this resonant frequency and splits the peak in a cute acoustical trick that permits the air inside the box to function as part of the speaker (feeds the back wave of the speaker to the front at the right point). This lowers the natural resonance peaks and introduces two smaller peaks instead.
Now the resonant peak shifts left (lower) allowing operation down to that point. Such a box does change the way a woofer sounds, just like adding mass, and to some it appears hollow or muddy. Such a box does allow deeper bass response than a conventional sealed box.
A.N. Thiele and Richard H. Small extensively studied the measurable electrical and mechanical parameters of speakers in free air and developed a set of data (commonly called the Thiele/Small parameters) that accurately predict the behaviour of a speaker inside of any kind of enclosure. Subsequent computer programs were created that take this data and apply it to a box simulation to predict the response of a speaker inside of a certain type of enclosure (a great application for a computer).
Now that you understand this a bit about boxes and resonance, you can do a simple test to see how low your speakers can respond without any electrical measurements or sweep frequency CDs. Take off the grill and gently tap the woofer cone. The note you hear is the lowest note your speakers can reproduce.