[ypana3]

I've heard a lot about how certain headphones need a certain time to "break in", but I have a question that has been bugging me. If I had two headphones of the exact same model (let's say the AKG K701 because they need a very long time to burn in), and one was playing bass heavy music for 300 straight hours, while the other was playing bass light music, would the headphones playing bass heavy music have a stronger bass response, or would they both sound the same? If they sound different, how big would you say the difference is? All help is appreciated.

 

[jaddie]

Been discussed to death already.  There is a lot of doubt that burn-in/break-in is real with every headphone, or to say it better, has a measurable effect in all headphones.  
 
The only way to answer your question is to get two sets of K701s, make initial tests and measurements, break one set it for 300 hours with the break-in signal flavor-of-the-day, then test both sets again.  Nobody's published the results of this test, and it may not have actually ever been done.  But until it is, we can't answer your question.
 
The closest answer is here, it leans to "no significant effect" but is actually somewhat inconclusive:
http://www.innerfidelity.com/content/evidence-headphone-break

 

[xnor]

Dynamic headphone driver burn-in? I doubt there's anything but very small changes.
 
Can old earpads cause differences? Yes.
Small placement changes causing big FR differences? Yes. This is not directly related to burn-in but it's completely ignored by people who say they hear huge differences.
Expectation bias and getting used to? Yes. Again, usually ignored.