6.5 vs 8 vs 10 woofer

timlub,

That is a pretty nice and concise description of Thiele-Small parameters.  Of course design and implementation are vastly complex matters even when one stays within the proscription of these parameters.  

@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.  

Too many permutations here to have a valid discussion. For example, a bi-amped system with electronic XOs allows for a dedicated amp to control the bass driver and not need as much passive XO at the speaker level. 
Then there is amp design, driver quality, damping factor and slew rates, etc. 

Douglas what would you buy?  Are you a dealer. I think this discussion makes sense because a lot of us are confused on this subject.

I thought he was basically asking a question rather than trying to tell us what is best.

I think we would like to hear what you have to say.  This is a question many of us have.  This also depends on whether you are using a pair of subs to fill in the lower frequencies.

I am listening to right now, a 15 that is 95db sensitivity, but only goes down to 38hz

~~~~~~~~~~~~~~~~~~~~
I would have figured a  15 goes BELOW 20hz.. Ok so the inner core also makes a  huge dif on how low a  8 inch will go.

Look Vox/USA just reached out, So Vox is back on the table.
And thinking things over,,although I figured a 10 will go into the low 30hz's, and the 8 low 40's, might not hold water. 
The 10 perhaps will go lower, however I've been living with dual 6/5's and have missed the lower bass for now 20 years, A 8 super high 98 sens will deliver the clean tight low, say maybe low30's, and thats allright with me. The Vox 8 looks to be ~~meticulously  hand crafted~~ and this may result in a  finer resolution of 100hz-say 1500 hz;s where most of my classical music falls. 
So it looks like the Vox 8 will be the next step in this FR/High sens journey, which is all completely new to me.
Just got word from the Vox /USA dealer, he has employed all sorts of PP tubes with Vox and has nothing to complain about.
So ~~Ideally SET  ~The Perfect Match~~, Due to finances (Defy upgrades well over $1k)  SET is off the table.
Now I have to go ck my topic over on amps, see if anyone has a few ideas how i can tweak,  the Defy to better match a  high sens speaker.
See you guys over there.