Pass Labs XS 150 Mono AmplifierLike the Xs 300, this amplifier was developed through constant and thorough innovation. After years of SIT device experimentation, followed by deployment in Nelson's First Watt designs, circuit modification captured the unique tube like sonic footprint of SITs from much higher output MOSFET devices used in the Xs series. The result is unparalleled, fatigue free listening enjoyment. Indication price per pair.