The M3 Triode Master is special version of the M3 designed for use with vacuum tube amplifiers. Tube amps typically exhibit higher than normal source impedance, which can cause frequency response anomalies when driving most speakers. SET amplifiers typically have the highest source impedance (1-4 Ohms). A relatively non-reactive speaker load is needed for best performance from such an amp, meaning the speaker needs to have near flat impedance curve. The M3 Triode provides a smooth 10-12 Ohm load to solve this problem, allowing your tube amp to sound its best.