There is one sub using 8" drivers that breaks a lot of subwoofer rules. It is the GR Research/Rythmik OB/Dipole Subwoofer. Three 8" woofers per sub, mounted not in a sealed or ported enclosure but in an Open Baffle "H-frame". Exceptional sound quality. The sub is also available with 12" drivers, providing greater extension and SPL.
By the way, Sterling Sound in NYC just added two more Rythmik F15HP subs to their mastering-house monitor systems, bringing their Rythmik F15 sub total to nine. Anyone not including the Rythmik Direct Servo-Feedback offerings in his/her sub search is making a huge mistake.