6.5 vs 8 vs 10 woofer

If you skip all ported options, you will probably notice that you like a larger driver. Have you listened to a dipole 15' or a sealed 15'?
My Barefoot monitors for instance uses 2*10' and yes, these are monitors.

Okay here’s may take, don’t be mean. I agree that an 8” driver is just about ideal for most music, vocals and instruments. But you do need a couple subwoofers to fill out the bottom end. I have found single full range coaxial drivers sound best to me. Multiple drivers tend to fracture the sound and you lose clarity of vocals and instruments if they are not aligned vertically AND horizontally . This is true if you are NOT trying to achieve concert volume levels! I have yet to hear a multiple driver system that does not have aligned point source drivers, match the clarity in the critical listening frequency range of say 70 Hz. to 5 kHz. I personally like Altec 601’s for the clarity and efficiency, they are what I want to afford. That being said, I would love to hear some Voxativ. You ABSOLUTELY need analog electronic crossovers and multiple amplifiers because passive crossovers I've heard sound bad. Started making my own electronic crossovers 45 years ago. So, Altec 601’s for the critical range, subwoofers below 45 Hz. and JBL 2405’s without the horns pointing straight up for highs above 8 kHz. with ANALOG ELECTRONIC crossovers and 3 stereo amplifiers. My ears have told me over the years that more is not better, just louder.

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

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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.

Yeah, because 95db is an extremely high efficiency 15" woofer and the problem here is that high efficiency woofers have to be engineered different and they do not have a "free air resonance" frequency low enough to do bass down at the 20hz point.  The woofers that do have a low enough resonance will need to be heavier in weight with a stiffer spider and therefore will not have as high of efficiency.  It's all a design compromise and we are dealing with physics here (no speaker manufacturer can give you everything on a silver platter with just one driver).  $$$ doesn't necessarily mean "best" or "better".

You can't expect all woofers to be your perfect dream.  Everything is a compromise. 

I've had discussions with mozatfan in the past where I suggested Markaudio drivers (he was on a full range driver kick), but he apparently poo-poo'd these drivers because they were British and he thinks British speakers suck.  At that point, I lost interest in discussion with him.

Full range drivers are always a complete compromise anyways where you sacrifice some aspect of the sound/characteristics to get what you want.  It is actually incredibly difficult to engineer and manufacture a full range driver that really works well throughout the entire frequency range.

He also seems to try to do "best speaker" arguments by selecting drivers he thinks will prove his point.  I do agree with others here, I think we have another Kenjit.

6.5 vs 8 vs 10 woofer ?
. . . uh, mine aren't round.

I've got 4 pairs of full range speakers with dual 7"s, 10", 12" and 15" woofers.  Of these, the 15" Tannoys have the LEAST amount of bass. The ported 12's go the lowest, but the 10's are in a sealed cab, so they roll off more gently.  The dual 7's are lightning quick, but don't have the 'gravitas' and punch of the larger woofers.  BUT, since the OP has declared that 8" is the ideal size, I guess I will just dump all these great speakers and find some with 8-inchers.