I fear in yesterday's post I may have lost a few of you: too technical, too many drawings and things to wade through. My apologies. Let me try again.
The premise I started with was a single device amplifier, like a SET amp, having only one tube per channel, cannot produce linear audio. This means the audio it produces has distortion. That distortion is euphonic. Many like how it sounds.
The distortion I refer to is caused by the very nature of the design itself. That design cannot properly perform the needed duties of a power amplifier when linear performance is the expectation. I think what I left off yesterday was to describe what those duties are. The job of a power amplifier is simple: push and pull the speaker drivers back and forth in perfect synch with the input signal.
We remember a loudspeaker works as a piston for the air. The driver pushes forward and compresses the air in front of it, then returns to its resting point and does the opposite: decompresses the air by sucking inward from its resting point. Back and forth, over and over again, the driver moves, hopefully, in perfect concert with instructions from the source of music, your CD player or turntable.
The problem with a single device amplifier is that it can only push the speaker driver forward with any degree of perfection. It cannot perfectly pull the speaker back. Think of it like a car with a broken reverse gear climbing a hill: step on the gas and the car charges forward in lockstep with the pedal. But try and go backwards without benefit of the engine and you are at the mercy of the slope of the hill. You give up the control and synchronized movement afforded by the engine and gas pedal.
The fact that a SET amp works well in one direction but not in the other means the output of the amplifier does not faithfully follow the input. That is the definition of distortion.
I don't mean to pick on SET amps. I actually like the way they sound and enjoy the warmth of the distortion products as much as anyone loving music in their home. My purpose is not beating up on the topology but to help explain how and why such things occur in design.
I hope this sheds a bit more light on the subject.