best sq 12 sub

Where can I read what you've read?

It does just what aznboi3644 says.  Almost all subwoofers on the market use an overhung motor topology which gives a parabolic BL curve to varying degrees depending on design.  XBL^2 is an ingenious design that allows a short coil to transverse two gaps to achieve a large excursion for the given coil height while achieving uniform BL throughout it's movement.  This linear BL results in drastically reduced distortion, especially within the 3rd order harmonic where distortion is most annoying to the human ear.

You mention JL, and throughout their line they use a standard overhing motor.  However with the W7 they modify the overhung design by drilling holes in the pole piece below the gap, greatly reducing stray flux and achieving flat BL.  That sub has very low distortion as well. 

Here's some tech papers about it:

http://www.adireaudio.com/Files/XBL2DetailsPaper.pdf

http://www.adireaudio.com/Files/XBL2TechPaper.pdf

http://www.adireaudio.com/Files/XBL2ExecSummary.pdf

And here's some DUMAX reports of different subwoofers to show their BL curves:

ID MAX: http://www.adireaudio.com/Files/IDMAX.pdf  This uses a standard motor topology

Audiomobile MASS: http://www.adireaudio.com/Files/DUMAXMASS2012.gif  This one uses an underhung motor topology

Digital Designs: http://www.adireaudio.com/Files/DDW6.pdf  This one uses an evenhung motor topology

JL W7: http://www.adireaudio.com/Files/JL-10W7.pdf  Notice how flat the curve is.

Brahma: http://www.adireaudio.com/Files/BrahmaDumax.pdf  As you can see there is little change in BL throughout excursion.

Okay Steven - I've read through a bunch of that stuff - very informative.  I appreciate you taking the time to post up so many links.

So the idea is then - just to try to distill from that tech paper and exec summary - is that this XBL motor structure is a mechanical change to the layout of the motor which produces a more controlled BL throughout the range of excursion - which keeps response very controlled throughout a range of output levels and frequencies?

When you say "parabolic BL curve" what you are saying is that the woofer's BL changes with output and excursion - producing distortion at higher excursions vis-a-vis the traditional, overhung motor structure of a standard woofer?

The first part is true.  But response in reguards to output and frequency response will be effected by other aspects of the speaker design as well, along with the enclosure used.  The XBL^2 motor will have some effect though due to it's higher excursion capability allowing for lower frequency extention and lower harmonic distortion resulting in less unwanted signal above the fundamental frequency.

A parabolic BL curve refers to the changes in BL with excursion specifically.  It is the change in BL when the cone moves from rest, whether forward or backward.  You can see this from the measured BL at different points of excursion on the links I posted above.

From the application notes, that thing looks like it'd take about anything if it was enclosed properly.  I wish I had the space for these they look so vicious - they'd be impossible in my car.

Why no 10-inch version Steven?

Power handling as always will be application dependant.  The subwoofer page offers different applications and recommended power handling capabilities for each. 

The Shiva-X and Tempest-X are both based off of the original Shiva and Tempest subs that were so successful at Adire Audio.  Adire didn't offer a 10" verson of those subs, so none is offered now.  Plus 10" subs really don't sell as well as the other sizes.