The M2 discrete operational amplifier is a low noise, high slew rate, low quiescent current, low distortion circuit. It can be used in all traditional audio amplifier configurations.
The differential input stage is a traditional bipolar junction long tailed tailed pair. The emitter current source is a high value resistor as opposed to the active current source arrangement that is found in many modern designs. This was a design decision made to retain a pleasant distortion profile. The input stage is fully differential, with two matched current sources loading the collectors in order to improve the input voltage swing. These collector current sources share their base bias with a nontraditional voltage amplification stage utilizing a cascode topology. This arrangement simultaneously increases DC gain and reduces total harmonic distortion. A quasi-complementary (Sziklai and Darlington pair) output stage provides the current drive.
All of these characteristics add up to a very low distortion amplifier that has dominant even-order harmonics, even in higher power output regimes. Clean, warm, and solid - the basis of the Iron Age sound.