SQ and size of the driver depends on a few things.
SQ suffers as the size of the driver increases, but that is only a general rule of thumb.
If a driver is designed properly, the sq will not suffer as it increases in size.
I have heard that the brahma 15" only very marginally suffers in sq compared to the 12" version -- means it's well engineered if that is the case!
I have also heard that the aura 1808, has superb sound quality.
Any subwoofer driver will sound bad if it has an improperly made enclosure.
It is absolutely imperative to have an enclosure designed well to have good sound. Talk to audiolife about his 15s and how good of sound q he can get out of them.
Some drivers to look into for sound quality AND spl
adire audio brahma www.adireaudio.com
image dynamics IDMAX
resonant engineering XXX series, www.respl.com
I'm sure I'm missing other great subwoofer drivers, but can't think of any.
DEQ-P9 (30band eq L/R, time alignment, 36dB/oct 4 way crossover)
Soundstream Tarantula (for subs)
Precision Power PCX-4125 (for horns and midbasses)
Image Dynamics CD2 comp mini horns
Scan-Speak 7" 18W/5535A midbasses
Subs still pending.