Zero feedback?Feedback is the act of taking the output voltage, attenuating it and applying it back to the input, out of phase with the incoming signal, so it can act as a correction voltage. SETs usually do not use it because they rely on the linearity of the output tube which is a triode and thus fairly linear.
Higher ordered harmonic distortion? Anyways, I am adding your input to my ever increasing list of things I need to read up on. For now I am taking away that the more sensitive my speakers, the better for getting the most out of my amp.All amps generate distortion, and the primary means is harmonics of fundamental tones. When an orchestra tunes up, its usually to A on the music scale; 440 cycles per second. The 2nd harmonic of A is thus 880Hz (Hertz is another way to express cycles per second). The higher ordered harmonics are the 5th harmonic and above. The ear uses them to sense sound pressure and so is keenly sensitive to them. For this reason when they are generated they can cause the stereo system to sound brighter and harsher (since the ear converts all distortions into a tonality and the higher orders get this assignment) and louder since they are cues to the ear of loudness.
The more efficient the speakers, the better, especially if you have a lower powered amp!