Many music "genres" require overdriven amp sound which setting playing volume lower make lower or even prevent. Push pull Power Amp including Phase Inverter and HV supply can be very important where also Output Transformer and guitar Loudspeaker have big roles
All volumes/gains lower and playing quieter lower the distirtion. MV lower or prevent the distortion what Power Amp could add if pre amp output is high enough.
PA Blocking Distortion is "major overdrive distortion" and it can be heard as "amp character sound". Electrically BD is "bias shift".
PA High Voltage Sag what Tube Rectifier amps have more also increase distortion which setting Volumes lower.
PA distortion what come when loudest overdriven notes are inside loudspeaker "resonance peak" which then increase Screen current because anode voltage swings very low to less than 100V. Electrical explanation is "loadline pass G1=0V below the knee". Searching that bring articles and analysis etc...
Output Transformer Core Saturation is one distortion which heavy overdrive can produce. It increase HV sag when power tube or tubes begin to pull anode current thru OT primary resistance instead of primary impedance. Push Pull tubes imbalance can make this happen earlier when primary currents come different.
Flyback Pulses is one overload phenomenon but obviously it is so extreme it is not wanted or needed. It can be seen using good oscilloscope and it look interesting but also scary?
NFB is interaction which I don't fully understand when it interact with guitar loudspeakers pequliar frequency/impedance response. When circuit has Presence/Resonance I have not done enough measuring to say much but it also look very interesting.
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