please some serious answers....
The rule of thumb is a hundred hours and two hundred hours is better. What's happening is things are loosening up and losing their stiffness so they're more responsive. Gives better extension into the bass and treble and the highs are not as harsh, less clipping to the ears. Nothing more than warming the muscles up before you grab on to the bar. The above applies irrespective if the IEM's are twenty dollar specials or uber expensive customs as all IEM's need some break-in time.
The problem with listening to IEM's during the break-in or burn-in period is, as the changes take place (and they do,) the changes are subtle so as the changes take place, you're not aware of the changes like a frog isn't aware of the changing temperature if you start the frog out in a cold water pot. If you want to be aware of the changes, you need to listen to them new and make notes. Then put fifty hours through them, listen again, making notes on the same piece and compare your jotted down notes. Let them burn-in for another fifty hours and put them on once more. Compare notes for a second time; new, fifty hours and at a hundred hours. A word to the wise, the less expensive IEM's are going show improvement but sadly no amount of burn-in will convert them into expensive sounding IEM's.
Some folks don't make note of harshness when new and others do. I know I do. Well, that's none of mine as that's theirs. A lot of dissatisfaction is because the user made a snap decision, based on an unbroken in piece of gear. The point, a lot of the break-in process is a personal and subjective evaluation process. Kinda like listening to a Tom Waits album; which I'm doing right now.
As to the burn-in process and the what music question. Anything is good as all you're doing in real terms, is giving the IEM's a workout. To me, the hard part, is not putting them on and using them while breaking them in.
"Wha-da-ya-mean? But I bought em to listen to!"
FWIW, I'm currently going through the break-in process with a new set of IEM's. I'm not making notes or anything like that as I've broken in enough stuff to know that it's all good. All that matters is what comes out of the transducers after the first hundred hours as I could care less what they sound like during the first hundred hours. To be honest, I can't live with the next hundred hours as I'm not patient enough to go through a second hundred hours of burn-in.
Hope the above gives you some insight to your OP questions.
