WOW! A lot to think about here. Of course the chain makes a difference. Marathon listening sessions? I checked your public profile. And I didn’t find you in the intros. What do you do for a living? Does it involve headphones?
Otherwise, my first thought is about the patient that went to his doctor, demonstrating “It hurts when I do this!” and the doctor replies “Well then don’t do that.”
Your low fatigue risk category is interesting. You’ll note de-emphasis of high frequency like cymbals. I’m not sure why male vocalists would not fall into this category - Mel Torme, Sinatra, Pavarotti. But “Margaret Thatcher Sings!” That’s fine. [humor alert] I find the Eagles to be fatiguing. One more Hotel California and I’ll get nausea.
Inherently fatiguing. Poor sound setups are fatiguing. Poor live venues and performances mixes don’t have to be recorded to cause serious ear fatigue. Add amplification, and it’s always worse. An orchestra in a theater with poor acoustics will be bad, but any electric/electronic instruments always seem worse to me. I’m talking live, not recorded here.
Rapid changes in volume - do you find a good Mahler recording to be fatiguing? (That’s a real question).
I find intentional noise to be fatiguing because well done it requires concentration. Think Paul Hindemith’s industrial music or the Beatles “Revolution Number 9”. I like the Orb from their middle period (Cydonia) and don’t find their mix quite as fatiguing - but they don’t have too many rapid volume changes.
I could go on with questions here. But getting to your point about the hardware chain. My favorite headphones are electrostatics. But I think the least fatiguing are Planar-Magnetics from the limited selection I have heard. The highest quality dynamics I own are Sennheiser HD-580s, and they will cause more fatigue than the HiFiman HE560s. I understand your point about lower end Grados. They have a signature clarity in and perhaps emphasis in the frequencies you appear to be most sensitive to.
One of the interesting Moderate category is “Distorted Electric Guitar”. I would be curious if you find Jimi Hendrix to be less fatiguing than other artists. From other reading, I seem to recall that typical musical instrument speakers at least in the past - had high inherent distortion. Jimi rather famously used to output his guitar through Leslie’s and not regular guitar speakers. They have an entirely different profile and sound. All of the distortion you hear from Jimi is that which he really tried to create - including the feedback.
I am not at all certain that analog sources are inherently less fatiguing than digital. I find that most of my analog was done in an analog chain, but I don’t know about remastered recordings. I sometimes have an old vinyl album and a new remastered one that probably has digital in the mixing, and the new one sounds overall better. Generally, I find live and analog sources to be less fatiguing than digital, but unless you are at an outdoor festival, you don’t have marathon sessions, and even there, bands change and there are less intense sonic times.
Let me go back to your first post. “I sometimes listen to headphones for long periods, and regularly upgrade or side grade or down grade in search of comfort over several hours.”
I think that the break and change of signatures reduces fatigue in itself.
