Grado Signature S550 HeadphonesS2 Driver Tuned for Musical Warmth and Performance At the core of the Signature S550 is Grado's 50mm S2 dynamic driver, tuned specifically for its all wood enclosure. Rather than pursuing an entirely new driver architecture, Grado focused on optimizing the interaction between the S2 driver and the acoustic properties of the housing. This approach yields a presentation that is warmer and more forgiving, yet remains fast, detailed, and spatially open.