its been pointed out that the 1st post had some basic rhetoric errors - a false equality of comparisons between links in the audio reproduction chain re "audibility"
headphone all vary by more than DBT/ABX thresholds - we know they "sound different" - measurement of output sound frequency response alone is sufficient to establish that differences are audible
wire/cable electrical interconnect electrical characteristics can be calculated, measured - for most situations in headphone audio reproduction their interaction with normal equipment does not give changes in frequency response that would be expected to be audible - often by margins of 100x
some don't seem to get the basic idea - you have to have numbers, sensitivities, knowledge of thresholds - just because every physical assembly has (possibly microscopic) different properties and some differences in some properties can lead to changes big enough to be heard doesn't mean any difference whatsoever can be heard
tubes made at the same factory can differ in critical electrical characteristics by >10% - vendors measure, charge extra for matching tube's gm to even 10%, even dual triode tubes - both sections in the same envelope can have Vt difference up to 2 V
in many common tube circuits, "normal" new production parameters range for the same model tube, and service life drift, wearout differences can have effects on amplifier response that are clearly above DBT/ABX audibility thresholds