With regard to electronics, it appears the author has conducted some characterization and managed to convince himself subjectivists are deceiving themselves. I could say the same about objectivists.
First, no two amps will ever measure the same - even ones from the same production line; unless the parts for that line are screened to strict standards.
Second, even if two amps measure the same, the devil is in the details. For example, identical THD+N for two amps does not mean identical harmonic components and magnitudes that comprise the THD+N figure.
Third, the details may matter depending on the system under test. Two competently designed amplifiers, that "measure the same", may sound different because the seemingly trivial details - which may not be well understood or characterized by the designers - heavily influence the sound signature due to their interaction with the specific characteristics of downstream equipment.
That equipment consists of cables and speakers that are a complex load of resistance, capacitance and inductance. Once out in the wild, an amp no longer encounters a resistor that is used for lab testing. To add to the complexity, that resistance, capacitance and inductance is not constant. It is ever-changing as the speaker is playing.
Getting back to my third point, if the amps sound different, one has to probe the system to understand how the amp and downstream equipment influence each other. Measurements have to be taken and then correlated to subjective observations to understand the relationship between system behavior and subjective impression. To my knowledge, this has never been done.
With the above as context, what does the author mean exactly when he writes "...amplifiers must have high input impedance, low output impedance, no frequency response anomalies, and be at all times operated within their voltage and current capabilities in order to sound the same"?
I think there's a lot we still don't understand. For example, what amplifier characteristic causes a lack of transparency in the highs? Could it be high frequency and phase distortion, however slight, that goes unmeasured?
Then there is that which we do understand but ignore either consciously or unconsciously. How many reviewers drive their amp into (slight) distortion but don't know it? How would they know? Slight distortion can be misinterpreted as ill-defined imaging or a reduced soundstage. Perhaps one amp doesn't distort and the other does even though they both have the "same" specs. One might be slightly more performant with the same speakers and cables because it has slightly lower output impedance or open loop gain (which the manufacturers typically don't spec and some may not even measure).
We need more science to appease the objectivists and subjectivists.
