I think you missed the question about what the way is you're able determine between good and excellent amps are though? All you said was "if you have a decently engineered amp with enough power, it comes down to the headphones or speakers".
I never said I could tell the difference between good and excellent amps. I said that in double blind tests people cannot tell the difference between good-excellent amps with enough power operated below clipping. So basically I said the opposite. I said if an amp is good to excellent people can't hear the difference between that amp and another good to excellent amp, meaning you can tell the difference between a good and a poor amp, but not between two good amps.
No idea if I personally can tell the difference between a good amp and an excellent amp--I've never tried it. But if people in double-blind studies can't tell the difference between two amps that are good to excellent, then I doubt I can.
"Oh and headphones are the bottleneck as you state in the start of your post. The only thing is, it's irrelevent because headphones are also at the point where differentiation between one or the other is basically down to FR(and this has nothing to do with quality as FR can eb corrected with EQ, but a bad FR does show lazy development of a headphone if that's something you would count, which I would agree wtih) and not actual other serious sonic qualities we somehow can't measure as some deluded audiophools claim (aside from distortion metrics sometimes due to headphone rig differentiations or headphone geometry construction that lead to undesired effects like resonanaces that may require a tad better gear to measure, or having parts that are created stupidly not taking into account things like contact with the ear and the driver)."
"Audiophools" are actually right about this. Amps and dacs can easily be reduced to measurements, but not headphones. It is a matter of taste whether someone likes one excellent headphone over another. I don't think I believe in the existence of "serious sonic qualities we somehow can't measure", but I also don't think you can place a score on a headphone that tells you how "objectively good" it is, whatever that might mean.
"...headphones are also at the point where differentiation between one or the other is basically down to FR(and this has nothing to do with quality as FR can eb corrected with EQ, but a bad FR does show lazy development of a headphone if that's something you would count, which I would agree with...headphones have also reached nearly the same status as DAC's and AMP's, audible transparency essentially".
This is objectively false. Notice lack of a headphone ranking chart on this site with corresponding measurement calcs. You can take frequency measurements using headphones on a dummy or whatever and try to assume a flat measurement is perfect and take into account the ways in which the sound bounces off your head making it less flat. But at the end of the day, there are too many variables and too many assumptions that have to be made. You end up with a squiggly line on a piece of paper that doesn't mean shit about whether you will like one headphone vs another one.