Sleek, solid-profile and modern, the Johnblue hydra headphone amp is an extremely high-quality headphone amp designed to drive any headphone with an impedance rating between 30 ohm to 400 Ohms. Of course, the audio synergy with beyerdynamic dt-990, Akg 601 , Ultrasone 2200 and Grado SR-25, is awesome but the hydra amp effectively drives headphones from any audiophile manufacturer with clarity and musicality.