This paper investigates different methods for improving the power efficiency of Class-D audio amplifier output stages. A standard half-bridge design is analysed and the effects of using Gallium Nitride (GaN) and Silicon Carbide (SiC) devices are considered. A new 2-phase interleaved output stage is proposed, and the advantages of this are discussed. The effect of the output low-pass filter parameters on power loss is also analysed. Simulation results of power efficiency of all of the proposed designs are presented. It is concluded that there is significant room to improve on the standard half-bridge Class-D amplifier design.