Lets Talk About Fatigue

Personally I don't get fatigue (guess I'm lucky) but as far as a software crossfeed goes, there are several plugins for foobar. The thing I like about them compared to hardware crossfeeds is that you can tweak them. I agree that a crossfeed can reduce SQ, especially in the bass region, but if I tweak the settings its perfectly fine. Not sure if it will help with fatigue, but I've heard it does so its worth a shot; especially when its a free plugin 

I'm generalizing (of course), but fatigue as I understand it talked about on Head-Fi is usually a result of a particular element or overall presentation of the sonic signature of an earphone. You can have peaky treble, which accentuates sibilance, which can be fatiguing (and physically painful, requiring painkillers). You can have a bump in the upper midrange, which creates forward, "shouty" vocals and instruments, which can be unpleasant to listen to for extended periods and which encourages lower volume listening. And, apparently, you can have extremely impactful and present bass, which can cause fatigue and headaches (ljokerl has mentioned bass headaches multiple times). I've never experienced this last one, but that's mostly because I don't own any bass-heavy equipment.

As to what it actually is, though. Hmm. I'd say it's another case of audio people appropriating the closest descriptor they can find in the language to something that doesn't really have a proper word. If you find something about a headphone's presentation fatiguing, you'll be encouraged to listen for shorter bursts. This is like sprinting, I guess, or engaging in grueling exercise. After a short, intense stretch, you're tired out. Thus, listener fatigue.

It's a nebulous term, and it's entirely possible different people mean different things by it.

I personally tend to get fatigued very easily from treble peaks and driver ringing.

Take for instance, the LCD-2 rev1, they have a neutralish/darker overall balance, but they were still quite unpleasant to listen to for any moderate length of time. I think it's because of the driver ringing near the sibilance range. Audeze had retarded QC at the time, so I may have been listening to a really bad pair when I had them for a few days.

On the other hand, the SR-007 has a very similar overall frequency response, but they have no trouble spots, so I can listen to them all day without a hint of fatigue.

I think the brain has a lot to do with it as well, because going back to my first decent headphone, the AT AD700, I can't stand them for five minutes now, whereas I used to use them for hours upon hours every day (probably because I thought sharp and piercing treble was 'normal'). 

Treble gets me, too, though upper midrange can be pretty bad as well. This is doubly annoying because I like present and well-extended treble (though not the shimmery, overly hyped kind), which basically sets up a minefield for any headphone that hopes to satisfy me. Present, well-extended treble often equals peaky treble, and that's a big no-no. I'm very sensitive to heightened sibilance, and without EQ I'd probably find most of the headphones of my preferred sonic signature to be unlistenable for any long duration.

I forgot about ringing. There be pirates sailin' the high seas combattin' that effin' ringin'. This probably explains why some headphones are EQ'able, and other ones, even though you've managed to damp down the peak, still manage to cause fatigue.

Fuaaaar I hate sibilance and really sreechy trebs. Makes me cringe. Maybe fatigue could be the overuse of the eardrum or something from certain frequencies whether it be bass, mids or trebs just like how your muscles fatigue after a long workout or after doing the same thing over and over again, or just overexposure to the same frequency and maybe that's how you get fatigued if you're listening to something with a big ass spike somewhere along the frequency range.

In my experience, fatigue comes from distortion.

I can listen loud with my Stax earspeakers for hours because they have a natural sound.

Instead there are other models that I have to play at low volume for not hearing too much of sound distortion.

You definitely have to add distortion to the list, particularly high-order and odd-order harmonic distortion. This is not something you can EQ away.

Interesting point OP makes, referencing physical fatigue. I >do< feel like I get to my fatigue level much faster after coming home from a long day at work - as opposed to tossing on some tunes in the middle of a day off.

The poster above my first post said they don't get fatigue, not me. I do experience it when I listen to a headphone that's peaky in the treble, or somebody talking through a really poor microphone (sibilance and harshness). There isn't really much you can do to "get rid" of it once it comes about, short of painkillers (if it gives you a headache) or turning down the volume or taking a break. Actually, taking an occasional break from listening is good advice in general, since fatigue can occur cumulatively over time.

As far as ringing goes, it's basically a phenomenon whereby the driver continues vibrating in certain frequency bands after the transient is over. In other words, when the driver is supposed to stop moving, most frequencies stop sounding, but certain frequency ranges continue after the others have stopped (sometimes for many milliseconds). This often corresponds to treble peaks but can also appear from a seemingly flat area of the response. Ringing can be detected using cumulative spectrum decay (CSD) graphs, also known as waterfall plots. Basically, these measure the frequency response over time, setting up a transient with equal energy across all frequency bands, then measuring the nature of the decay after the transient is stopped. Ringing usually appears as long, thin trails clearly emerging from the gentle downward sloping wall of the surrounding frequencies. Generally, the longer and sharper the trail, the worse (more audible) the effect.

Study of this stuff in headphones is still in its infancy, but it's believed (and a lot of subjective accounts suggest) that headphones with lots of ringing in the treble tend to cause fatigue over time, even when compared to headphones with a similar balance and similar treble peaks but which do not exhibit significant ringing. The DT880, despite its bright treble, doesn't ring significantly, so EQ works wonders on it. The SRH440, on the other hand, probably rings at least around its treble spike ~9kHz (I haven't seen a CSD chart for the SRH440, but the SRH840 has some ringing, so I'm extrapolating from that), and, consequently, on rare occasions still causes fatigue despite my EQ curve.

Certain headphones make me feel more fatigue than the others. HD650, for example, never makes me feel fatigue.