A SQ subwoofer in a car for pure SQ can be **** near anything. You can barely tell the sub is playing in a SQ rig tuned for competition, most competitors only run their settings like that when they are being judged. It tends to sound better on most real music using other settings.
I don't know where anyone would say the DD subs are a beefed up SQ design. They are overhung motors, with very stiff suspensions and favor high BL product over BL linearity. Dan Wiggins years ago showed them with XBL^2 they could add quite a bit more xmax by using xbl^2, of course DD wanted nothing to do with it. I believe he tested a 9500 series and XMAX came out around 20mm. For a low end daily pounding SQ app, that's not exactly amazing, especially since the suspensions are noisy.
Only other thing I'd like to add is having a -3 point at 28hz in a car is NOT flat response by an stretch of the imagination. Cars are a small environment and that effects how much low end bass you need to get a curve that sounds flat. By sounding flat, I don't mean all frequencies the sound the same, but I mean sounding like RTA flat would sound in a more open environment. To replicate that in a car you need to be closer to +3 to +6db at 30hz depending on the cars size. The DD is about 6-10db's down from that mark tuned at 40hz playing at 28hz. It probably also falls flat on it's face above 60 unless is supercharged, but that's a whole different story. Anyway the Brahma's he alluded to, your correct couldn't play anything else good. Super low distortion from 50 to as low as you want, but inductance cuts them off above 60hz. The later gens Brahmas corrected this as well as the Tempest-X that was brought out much more recently. Most of the higher end TC subs use LMS tech, which makes them a bit like the brahmas unless the use a shorting ring like the LMS ULTRA or the LMS-R. I beleive the old TC-4000 and the Soundsplinter RL-S used LMS tech with no rings, low effeciency to boot, so no upper bass output from those, but very clean lows.
Point is you CAN have it all with a well engineered driver, DD subs generally aren't the ticket, but I haven't seen a supercharged one yet. The shorting rings should be a step in the right direction, but the suspensions, while bullet proof are hardly SQ.