6.5 vs 8 vs 10 woofer

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.   

@douglas_schroeder --

My favorites? None, if not set up well. Several, if set up well.
Over the years I am moving away from standard dynamic speaker towers. I am moving increasingly toward Open Baffle and Dipole (with or without subs). I can hardly stand the sound of a bass driver 8" or under; too earnest, too much popping of the driver when pushed. They just can’t handle LF and higher levels (Zero interest in arguing with people about this).

If you have poor amplification, you suck the life/quality out of the bass. If you have poorer cables, ditto. It’s a spectrum of performance, but also a spectrum of genres of bass technology - none of them definitively the best. YMMV

To me passive cross-overs, by and large, have a thickening and dulling effect on the bass (and the remainder of the spectrum), so active configuration is one step towards better performance here.

Going on from here I have no qualms stating that high efficiency 15" drivers are the best compromise up through the lower midrange, not least relieved of low bass reproduction when high-passed actively. Arguably the very best scenario in the power region - again, to my mind - is loading a proper 15" HE driver in a simple-fold horn (i.e.: not all too many folds), actively; this way the bass to lower mids will have an effortless "twang," smoothness, texture and dynamics that's simply unparalleled by any other bass principle that I know of. A "proper" 15" HE driver for this type of horn loading is rare though these days, if they're made at all other than by boutique, small scale manufacturers. Horn loading this way lends you ~105dB sensitivity, so the (15") bass driver cone will barely move at all, even at very high SPL's - not least when high-passed down low - and that's exactly what you want to achieve: less cone movement with high efficiency and lots of displacement area, not to mention via good air coupling. The only midrange/HF solution that can match such a bass horn is another, preferably large horn and compression driver combo, so there you have it; that's my preference, though it's not to say I can't enjoy a range of other well set up speaker principles.

However, it still goes to show that the typical love for smaller diameter, lower efficiency and direct radiating drivers comes down to convenience and what's most widely available; not a true choice per se. Who have heard a big, folded bass horn loaded with a 15" HE driver in their home - not to mention even bigger horn subs? Close to none, and so the predominant bias away from this segment of speakers is founded in conjecture or that it simply doesn't exist, and not actual experience. Added of course to the mere thought of having such behemoths placed in a home environment, that will no doubt scare most away. That, at least, I'm sympathetic to, though the lack of all-out and to hell with practicalities and WAF is still to be lamented. 

@Mozartfan
I was with you in the wideband hi-eff thread, but this 10" theory makes little sense. @auxinput gave you very important information. he's right. even a 15" hi-eff are limited in low bass due to the hi-eff design choice in building a hi-eff woofer. so a 95db 15" woofer need a BIG cab to make low bass: like 200L or more.

- no 10" wideband will reach 20khz smoothly. the rising response will be impossible to deal without compensations circuit to smooth that rising response

- a 10 inch wideband still need a light cone and relatively weak magnet= compromising low bass, and Ive heard a couple like the famous 10" vintage philips. they just dont do low bass

if you want a wideband and bass, look at FAST designs (fullrange-assisted-system-desing) at diyaudio. whole following there.

If you want wideband sound and real bass, you need subs. If you dont listen loud, get a 5, 6 " voxativ in a small sealed enclosure and try to make the wideband roll off naturally around 100hz, then integrate dual REL subs

hope this helps

PS: if you want hi-eff and real bass, go the classic way: hi-eff 15" woofer with a 6.5" hi eff mid and hi-eff 1.5" tweeter (seas t35 are 96 db) and go for a 1st order xo.
I’m building right now such a speaker with a Eminence omega pro 15", audax 170mo mid and hivi planar tweeter, 1st order series xo in a 200L cab. This system will be about 96 db efficient.