The speakers that you choose for your guitar amps will be the main contributing factor to how the guitars tone is going to come out sounding. However, tone is also subjective and often the tones that a guitar player actually loves comes from technical imperfections. The earlier amps used for guitars were equipped with cone shaped speakers and they didn't have any tweeters, by doing this, some of the less pleasant sounds were filtered out as well.
If there were no technical limitations with speakers then the tube distortion would be rather unpleasant and even gritty sounding this is why the better guitar amps even today still use a limited range of cone type drives and they still don't use tweeters.
Sound that comes from speakers will be determined by its cone along with a magnetic motor. The 12 inch sized speakers are usually the more popular ones for combos and four driver units but some guitar players do prefer a sound that is less bassy and they will then go with a 10 inch speaker. A lot of the cones for speakers are often made from a type of doped paper but others are made for different kinds of material. They can also be made with paper that is mixed with other fibers. The smaller sized speakers are usually chosen for practice amps and not often used for performing with.
Speakers have character and some of that character will come from the way the cone will flex when it is pushed hard and this is called “break-up”. The more thin and lighter cone will tend to break-up more quickly and will give more distortion and sometimes sounds compressed. The speakers with more power will often have heavier cones that let them stay rigid and will not break-up as easily.
In some of the cones put in speakers you will find a variety of concentric circles that are molded into the cones. The reason for this is to let the center of the cone move easier during higher frequencies. But this kind of design will also effect the sound of the bass.
If you need to replace your speakers in your cabinet it is important that they are replaced with speakers of the same impedance as well as the suitable power rating. It is also important to make sure that the holes for the bolts match up. Often manufacturers don't use the same type of layouts. Now, if the cabinet for the guitar is one that is open backed then there will be a lot of freedom for choosing the kind of speaker to fit in the cabinet, but if it is a sealed type of cabinet this may take professional advice about what the best speakers might be that would fit in the guitar cabinet. Speakers can be mixed and matched when there are two or more but the efficiency of each must be pretty similar or one speaker will end up dominating the other instead of having smooth balance between the two.