interesting comment : certainly the 15" will deliver more output, but do they also trump the same brands 12" in SQ? I would have thought not, as the 12" should be slightly more responsive.
And relately, wouldn't a pair of 12"'s then be better than one 15" - you increase the output, and gain a slight advantage in SQ? And gain the benifit of x.2 placement flexibility...
This is a myth. Smaller drivers are generally desired because of their wide dispersion at midrange and high frequencies, smaller box requirements, and naturally lower inductance and mass.
Since for an 21 and 8 inch driver, the passband (15 to 160hz or so) of a subwoofer will be equally omnidirectional, dispersion isn't an issue. Since we want to low pass them at 80hz, the rolloff and reduction in pistonic behaviour around 200-500hz is essentially irrelevant. And since box size only impacts us aesthetically, it does not indicate sound quality if well designed.
A well designed 21 inch driver will wipe the floor with a comparably designed 12 unless you need it working up high in a 2.5 or 3 way. The sound quality at bass frequencies, if anything, will be improved because displacement becomes less dependant on excursion, keeping the BL high. High BL means stronger control over driver motion, so keeping it high means consistency. Combined with a well damped enclosure and O would take great 15s for sound quality at bass frequencies any day over great 12s. Having a great motor is key. You can't put a 10 inch driver's motor on a 15 inch cone and expect control but this motor, LMS, is a tough to beat option, plain and simple. The 15 and 18 inch LMS drivers are just scary ;P and Funkywaves uses them too.
Bass frequencies aside, often we want a narrow dispersion pattern. Some speakers, as those such as JTR, JBL, Geddes, use very large, low mass pro style drivers at frequencies as high as up as 2khz for very controlled dispersion resulting in excellent inroom sound quality and high output levels... They're ugly though.
And you must know not-awesome women because that finish is sexy.