This paper describes a knee prosthesis that provides primarily-passive behavior akin to a state-of-the-art modulated passive microprocessor-controlled knee prosthesis (MPK), but also offers powered stance and swing assistance, which are layered on top of the passive functionality. The combination of both passive and powered behaviors is enabled by a unique electronically-selectable two-speed transmission, which enables the combination of very low output impedance and high resistive torques necessary for passive functionality, while also enabling the generative torque-speed characteristics suitable for stance and swing phase assistance. A control system is described that consolidates behaviors appropriate for a wide range of activities of daily living (ADLs) into six states, with each state providing adaptive torque behaviors appropriate for different phases of gait, whether powered or passive. Experimental results on an individual with transfemoral amputation suggest that the prosthesis and controller provide similar or improved passive behaviors relative to a commercially-available MPK, in addition to providing powered functionality commensurate with a powered prosthesis.