This paper presents a novel RF load pull test setup that allows for the presentation of user specified impedances at both the RF (Radio frequency) and IF (intermediate frequency) domain, to enable the investigation of linear high efficiency power amplifier design using measurement data. The RF load pull impedances at the fundamental, and second harmonic were set to produce a derivative of a class J amplifier to emulate a high efficiency mode of operation. The IF impedance up to the fifth IF product was then varied to quantify its effect on linearity. To enable this investigation a multi-sine stimuli was synthesized using multiple signal sources all phase locked to reference local oscillator. It was found that a greater than 10dB improvement in linearity was achieved by actively injecting a signal at IF that can be applied in an envelope tracking system. © 2018 IEEE.