Actually, the absolute 'best' quality 15" driver would be one using the LMS motor from TC Sounds/Audio Pulse. Sound Splinter (
www.soundsplinter.com) has a TC Sounds built 15" subwoofer using the LMS motor:
http://www.soundsplinter.com/rls15_subwoofer_information.html
The LMS technology provides for the most linear motors in existence by way of using traditional techniques to lower distortion (copper shorting rings, etc.) combined with an innovative variable density coil. The voice coil actually has different thickness as you move from the center resting position in order to account for the natural motor strength differences(BL) at varying positions. The result is a near perfect flat motor response regardless of 1mm or 30mm position of the coil from center rest position. The means super low distortion and minimum TS parameter changes at different excursions. However, these drivers are rather low in sensitivity; meaning that high power amplification is a must.
The AXIS drivers are one step down from the LMS drivers from Audio Poluse.
-Chris