Just a few sweeping generalisations:
(1) Unless you are paying oodles of money, front speakers will not be designed to reproduce frequencies at lower than 40Hz which have the same loudness as those frequencies above 40Hz.
(2) A speaker may be able to produce the same level of loudness (decibels) over a wide frequency range, say 40Hz to 20,000Hz, but at either end of this spectrum the loudness tails off. In order to give the manufacturers some consistency in how they define their frequency response, they are allowed to report what frequencies they can produce at up to 3 decibels below the average decibels over the main range. The rate at which the frequency will "roll off" below these lower and upper points, will depend on the speaker design. Some speakers will be designed to roll off their output at the rate of 12 decibels for each octave either below the lowest range point, or an octave higher then the highest range point. Some speakers do 6db, others higher like 24db. An "octave" is the halving or doubling of frequencies from any particular point. So an octave below 40Hz is 20Hz. an octave above 40Hz is 80Hz.
(3) As a result of (2), normal speakers are not adept at reproducing those frequencies in a film which go below say 40Hz. This is where the "rumble" and feel of a sound becomes more apparent than actually hearing the sound. Hearing changes from person to person, changes with age and with lower frequencies the perceived loudness changes with the frequency itself. So in order to get a true home cinema experience you need to supplement your front speakers with sub. Some people can "hear" as low as 25Hz, other can "feel" much lower even down to sub 20Hz. Depends on many factors.
(4) The power to reproduce low frequencies can be immense, disproportionately higher per decibel than at higher frequencies. Consequently it is advantageous to have a subwoofer take over some of the lower frequency reproduction that the speakers produce. Subs are designed to deliver power at the low end. The THX standard for multi channel sound reproduction is that an amplifier should be set to "split" the frequencies above and below 80Hz to the speakers and subwoofer.
(5) Now the 80Hz "crossover" setting in an amplifier is not a brick wall. The amplifier will "roll off" the frequencies, either side of the crossover, it sends to the sub and speakers. In this way there should be a smooth and level transition of sound that is produced simultaneously by both speakers and sub. It is fortunate that when frequencies get to about 80Hz (depends on people) they become difficult to locate. What i mean by this is that you could play a 80Hz tone from each of the main speakers and you not be able to tell from which speaker it was produced. So in theory and ideally, a sub's output should be heard and not "seen" as it were. It is there to supplement the main speakers sound-field for that all important "thump" in the chest moment!
(6) Because the THX standard is 80Hz (some prefer other settings depending on their front speaker capability), getting front speakers to go lower than 40 Hz (an Octave below the crossover) is somewhat fruitless as those frequencies will be routed to the sub by the amplifier. However, a rule of thumb and the fact of the mechanics of how an amp will "manage" the roll off of frequencies around the set cutover point, it is suggested the amps cut-over be set at twice the frequency of the lower frequency response of the main speakers. However, if you have the "new" satellite speaker type arrangement, these typically will not go much lower than about 60-80Hz. In this instance the crossover could be so high as to send frequencies to the sub which can be "located" by ear: not a good thing. So then you have a compromise between smooth and level transition between speakers and sub, and the ability to make the sub's output "disappear".
(7) Low frequencies that a sub produces say at 30Hz and below, have no regard for most barriers in a home, including brick walls. It is these lower frequencies that will permeate you entire house, and your neighbours if you have them. So you need to be careful when you choose to ratchet it up!
(8) The reason why you may have lower frequency producing fronts is if you are a bit of an audiophile and want to play stereo. Many purists will not touch a sub to supplement the speakers in this scenario. However, even if you get speakers which are "rated" at 40-20,000 Hz at -3 decibels, for home cinema the amp will manage these and the sub well for film reproduction. You can then use them on their own for "stereo"
Hope that helps a bit more without confusing you further.
This is a graph of the frequency responses of my speakers when the amplifier iss et with different crossovers: you can see how the 80Hz crossover reduces the frequency output below the 80Hz mark.
Conversely, her eis a frequency plot of my subwoofer showing the opposite effect on the sub output:
