The driver is adding a very subtle and pleasing harmonic distortion to the signal. This ends up boosting the harmonics above the normal range you expect to hear which creates a sense of fullness. When an entire song has each track featuring this warmth, you get an extremely full sound.

Devs, Prods & Tags: Babelson AudioFD2N

Videos by Babelson Audio
www.kvraudio.com/developer/babelson-audio