I couldn't agree more. I find it curious when some folks say that a headphone "makes" *some* recordings sound bad. I keep going back to thinking that some headphones just reveal how bad a recording actually is. Some mask it. Doesn't that mean that those are masking some of the defects? If faithful reproduction is what we want, those are not doing it.
Having said that, I find the 560 with the sources that I have tried and amps that I have used to be a bit hot in the upper midrange. I don't find them sibilant per se. But they really do reveal any microphone or recording information that highlights things like S sounds. It is not like the kind of harsh sound that I usually assign to sibilance... it is not painful or aggravating... to me. But if it is there, you definitely hear it.
I have got around 100 hours on my set. I typically do not really hear much change in most headphones from out of the box to several hundreds hours of use. I am going through the exercise in any case..
just as a new data point.
I picked up a Modi Uber just to try. It is definitely a bit more laid back than the ESS DAC in the NFB 11... this driving the Polaris.
I plan to play some more with speaker taps and some of my hybrid amps... I am also strongly considering the Jerg 1.5 mods..
He claims that the shelf liner disk with the hole in the center brings some, in his terms, "Welcome" treble reduction!
It's the same difference whether a headphone reveals imperfections accurately (making the song sound bad) or a headphone glosses over imperfections (making the song sound good). At the end of the day, taking pride in the fact that your music sounds like **** because your headphones are so good is a little counterproductive, no?
The real point to take away is that there are headphone setups that can be revealing without being harsh in the trebles. As you eluded, it's a combination of factors in the chain, but the headphone has the largest effect and it's the easiest to change. You can't do much about a bad recording.
I tried chasing the elusive perfect treble with my HE-4 with pad swaps and gear swaps and such. Eventually I realized it was a lot easier to just get a headphone that performed better in the trebles.