A transistor is supposed to be linear current-amplifying device. This is true to a point where the transistor is "saturated" - higher input current does not result in higher output current.
What's worse, when the input current is canceled, the output current drops low with an additional delay. In an amplifier this causes enormous distortion, the output signal gets full of high frequencies, which can even damage tweeters in speaker sets.
The way to get rid of this kind of distortion is to not allow the amp to saturate while in "limitation". The above was meant about bipolar transistors, FET's and tubes do not saturate. The tubes, however, are not linear, their amplification drops long before closing to the state of limitation, and they have high output voltage and impedance - it has to be transformed, which is another factor contributing to distortion.
This must've looked quite scientific, but I asure you I don't understand much about how the circuit works - in fact it is the three additional diodes and a capacitor in each half of the differential input stage. The fact is that it works. The author wanted to patent the circuit, but he found out he was not the first - we Czechs say he discovered America. Some firms like - I guess - Denon, NAD or Technics also have something like this.