I think burn-in exists, but as kuhchuk has suggested, it is your brain that does the burning in. Far more likely and probable explanation than your gear changing dramatically.
Thank you for the support, but I think the general concensus for the meaning of burn-in refers to the gear changing, not your brain adjusting.
No, it isn't.
At first, my HD598's sounded, well, quite bad. But now, after uncountable number of playing hours they are much more detailed, open and separates miles better. And yes, I clearly remember how they sounded first day.
In fact, almost every piece of audio gear benefits from burn in. Even electronics, cables, capacitors, let alone headphone membrane which mechanically vibrates..
Let's break this down one step at a time.
1.) You do not remember how your gear sounded more than a few minutes ago. Our brains aren't equipped to store exact waveforms for more than a few seconds.
2.) If your HD598's didn't sound "good" to you on day one, that's because you weren't used to how they sound. After your brain adjusted to them, you started to enjoy them more. There is also no evidence that "burn-in" does anything to change how headphones sound. People have tested this by taking a brand new set of cans and measuring the frequency response before and after hours upon hours of burn-in. Any "change" in the plots were easily within margin of error, and you wouldn't be able to tell the difference anyways.
3.) No, none of your gear should benefit from burn-in. Cables are the least likely to require burn in. There is no physical or chemical change that could occur in your cables to make them sound any better or worse (and that's assuming cables make any difference at all (which they don't)).
4.) The only change that could be made in a capacitor is it's slow degradation towards death. If you think you can hear this, you're mistaken.
Any piece of electronics these days
should be made with parts that are of high enough quality to work properly from day one. If and when a certain part does require some sort of "burn-in" time, this should be done AT THE FACTORY. If not, then the manufacturer is either A.) Doing a ****ty job and cutting corners, or B.) Lying to you so you keep the gear past the window for their return policy.
Another thing to note is that proponents of burn-in almost universally say their gear sounds
better over time. Surely, if burn-in were real, there would be people who thought their gear sounded great at day one, and actually disliked the changes in their gear after it was burned in. This is a damning piece of evidence in itself, even if you disregard the actual research that has been done about the topic.
And to wrap this up, let me use an analogy.
Do you believe that your monitor looks better over time? Does your router speed up? Does your computer crunch numbers faster?
No? They don't? Why not? According to you, even solid state circuitry performs better after some burn-in time. If that were the case, ALL of your electronics would perform differently over time, and not just negatively as parts degrade.
