6.5 vs 8 vs 10 woofer

@mozartfan  
I suggest that you study a bit about speaker design.  There is a lot more than spec's, but they do tell a basic story.  Look at the spec's on drivers that you are comparing.  
First Mass MMS... The amount of cone & suspension weight will definitely affect the sensitivity as well as how low a driver will play.  
Take a 95 db woofer that goes down to 50 hz add enough mass and you may get this thing to go down to 40 hz.  The problem is, when you add mass, the motor will have a tougher time pushing the mass and sensitivity will drop, in this case, it could be 5 or 6 db.  
Second Xmax.  In the case above, if the cone does not have enough excursion limit, it would easily bottom as mass is added.  Xmax is important. 
Third FS.  This tells you the resonant frequency that a driver plays on its own without a baffle.  It will also give you an idea of how low a drive may be able to go (depending on box)
Fourth QTS.  This tells you what type of box the driver is best suited for and how that driver will play bass depending on the type of box.  
Fifth VAS.  When tied into conjunction with everything else above, this will tell us the rest of the story of how low and how loud a driver will play as well as how flat its response is in a given box. 
In general (not cast in stone)  The larger a woofer is,  the lower the FS is and the higher the sensitivity can be.  
Still,  I have actually had an 8 inch woofer that we modified heavily, but in the end, this driver would play an F3 of 22 hz,  BUT, it ended up being 83db sensitivity and didn't have enough XMAX to drive it very hard.  
I am listening to right now, a 15 that is 95db sensitivity, but only goes down to 38hz.... Try to research the amount of air a 15 moves vs a 6 inch, even though both may go down to 40hz, what does driver size mean? Everything is a trade off, if I didn't add so much mass to the 8 inch,  it still would have gotten down to 32hz and gave me more like 86db sensitivity.  Look at these spec's,  do some research, get a basic understanding and you'll be able to make a better choice on drivers based upon your needs

@larryi    Oh Larry, you are absolutely 100% correct. I was trying to at least give him guidance on where to look for a basic understanding. He would never design anything from that brief description, but it is a start. Recently, I've been playing with how frequency response changes with impedance compensation. Where in the world does that fit in this discussion or how do you get there..... Just 1 step at a time.  I was trying to give him the first step.  

@mozartfan  I'm sorry, I don't mean to be rude, but you are clueless. 
You can certainly find An example of each of the above, but making blanket statements is naive. 

 Its ridiculous listening to a tiny 1.3 inch dome tweeter trying to gather up a full range orchestra in full power." 

 You are right it is,  that's why we use another driver below it and a crossover.  

tweeters reproduce harmonics mostly and are used over 2khz. listen to your music, turn a EQ on and highpass your orchestra at 2khz. not much energy over 2khz.

Ok , so there is no argument, you only listen to music up to 2000 hz, since anything over 2k is irrelevant because it is only harmonics. 

@ mozartfan   I really do hope that you look over the information that I posted about understanding speaker specs and what they mean when it comes to understanding a speaker.  One thing that I would say that I don't see regarded much on domes is Xmax.  1.3 inch domes out there currently have an xmax of .1mm to 3.0mm.  This makes a tremendous difference in how a dome translates music.  There is a tweeter fairly unknow that I truly love.  I posted it on a thread called what are your favorite tweeters.    If you'd like to pm me, I'll do my best to answer your questions and offer a better understanding of why some of your idea's are actually quite good, but others show a very poor understanding of how all of this works.   Good Luck.  Tim