I can help out a bit here.
The specs on speakers are very revealing and you need to use those to guide your choices.
Wattage, the most popular doesnt tell you jack except the maximum wattage the speaker can accept. Its like a fuse rating to me. So long as you dont exceed it it wont blow. Thats it. It has nothing to do with tone or loudness or responce. Simply match the RMS to the amp and you're good. An rms level too great for the amp will likely never allow you to get good speaker breakup, and RMS too low is dangerous for the amp and speaker.
Impediance (DC Resistance) simply has to match. Thats about it.
SPL - a major key element is the Sound Pressure Level for the given wattage. The higher the number, the louder the speaker. Guitar speakers are normally over 90, HiFi below.
For a low wattage amp using a high SPL of say 105 would mahe the amp maybe 10db louder than a speaker with an spl of 95. (not exact math here but you get the idea.
A HiFi speaker is tuned to a cabinet and can have a lower SPL because the air mass works for the speaker to enhance the low end magnifying the bass. Crossovers and mids/tweeters push the other frequencies so, so long as the outputs of each are properly attenuated, the full frequency of all match and you get a full range of sound. The woofer merely needs to be linear as volume is increased, and within its responce curve. The porting of the cabinet keeps the speaker from pumping against the air mass, and the placement and length of the port makes the air moving through it in phase with the speaker virtually doubbleing the speaker efficiancy.
HiFi speakers sound terrible for guitar. they are woofey and lack mids and always have too much bass at all volumes. Guitar speaker frequency responce is midrange, so resonant frequency and bass arent a huge factor. Neither are the cabinets in comparison to Bass Pa etc.
The responce is targeted between the bass guitar on the low end and vocals/cymbals etc on the high end. This gives a frequency range of say 150~6K at the highest extreme, 5K is plenty.
Cabinets dont have a huge effect on these ranges so I wont get into huge detail and get into figuring air volume/tuning crossover and all like HiFi, bass and PA stuff requires. Even the Marshall cabs when first built didnt use math and resonant frequency responce in their building, it was more astetics and luck/gestimation in their design but sufficiant air needs to be there to give nice bass tone.
Open backed infinate baffel cabs are often used on stage and placed parallel or in front of a drummer and act as monitors eliminating the need to pump a signal through the vocal monitors. This keeps the drummer on beat and eliminates the need for expensive PA equipment. Closed backs are usually placed further back on larger stages and use higher wattage amps. Higher wattage means bigger pro PAs and monitor sends so the open backed isnt as important as a direct monitor.
So we have SPL = Loudness - frequency reponce = max/min - RMS matching - Cabinet choice for venue
Next and the most important is tone targeting.
Tone incorperates the entire chain. For example, you have a guitar with warm wood, hot pup with extreme output at 3K driven with an amp that has strong mids driving a speaker that has a strong 3K peak. Result may be great mids and speaker breakup for playing leads but uncontrollable mids for playing rythum. Then take a low impediance fender pup with a full range for a nice jangle, put it through the same setup and it may sound warm with the mid filling in but you cant adjust the mids out to get that same jangley fender sound.
This is where targeting a speaker to the amp responce comes in. If I had an unknown amp I would test it by pumping pink/white noise in and testing the output with a frequency analizer. The adjust the eq and plot the changes for trebble mids and lows. This can be done with simple computer software through a sound card properly connected.
Say the amp was simular to a marshall. The first 3 graphs below show the amp has a 5K, 1K and 100hz sweep for the controlls. If I bought a speaker I'd want one that could handel those frequencies efficantly. If I used a speaker that rolls of at 4.5K the amp wouldnt have the sparkle it should. My trebble knob wouldnt work efficiantly and I'd loose a good deal od SPL/DB level from the amp.
I want to target the speaker to the amp so the tone controlls give a fairly flat responce to the sound when set in the center, and plenty of +/- when adjusted up and down. This allows me to adjust the widest ranges for whatever guitar I might plug in. I would use the amp responce and use the Speaker responce graph, plus the approximate cab size to enhance the speaker responce and match the tone I want.
Now I've been leaving out the speaker design till last. Its very important too. The paper material, thickness, stiffness, spider, magnet etc all have different tonal effects. If I had an old tube amp with a farty low end that sounds flabby and wanted to tighten it up I'd likely go for a speaker with a stiffer cone to tighten up the low end. If I wanted it to break up smooth earley on the volume controll might try low wattage alnico speakers in multiples. If I want clean chords with littel breakup but punchy, I might go for a stiff coned ceramic speaker. If the amp lacks low end punch I may use a speaker with a looser cone and spider. If I want more rip, I'd use a thin paper cone. If the amp needed more mids I'd look at the responce curve, target the mids lacking and choose a speaker that was hyped in that area, if the amp sounded small and middely, I'd do the opposite.
Theres alot more to it and experience and choices of speakers are important. You just need to be sure you have good controll of the frequency spectrum between the bass guitar and vocals/cymbals on stage. Anything above 5K creates competition/masking of the vocals. Most guitar amps dont reproduce sounds in that range anyway so piercing hyped highs would only be an ice pick in your ears anyway. Got a deep voice, target the amp with less high end, have a female vocalist, push the high end a littel. Deep bass guitar, less low end so the band doesnt sound muddy, slap bass, maybe a littel more low end to compensate for the lows missing on the bass. (or you could do the opposite and keep the guitar in its own range that doesnt mask the bass. The choice is yours. This is about it for now.
Heres the specs on one speaker and you can analize them yourself.
Heres one I pulled from Parts express that has all the specs. The speaker has an SPL of 100 which is fairly loud, better than say Jensons. The responce curve has a big spike at 2~5K. If I wanted to get say a fender amp to sound more like a marshall, this might be a good choice.
http://www.parts-express.com/pdf/290-840s.pdf
http://www.parts-express.com/pe/showdet ... er=290-840
If I had a fender guitar and amp with weak mids I might choose something like this to boost the lower mids
http://www.parts-express.com/pdf/290-804s.pdf
Again a lower 2K bump
http://www.parts-express.com/pdf/290-808s.pdf
A 1.5K notch and plenty of upper mids
http://www.parts-express.com/pdf/290-816s.pdf
The difference in tone between 1~5k is very great. A small bump can easily be detected by our ears whereas a change in frequency at 10~20K is limited. An amp with a 2K bump is going to sound very different than a 3k, 3.5K 5K etc so theres quite a range in the middel for speaker matching once you know where the amp is most efficiant at.