Consider this like the first time you used a baseball glove the leather was hard and stiff and the more you used it the softer and easier it was to use, even the sound of the ball hitting the glove changed after the break-in.
The break-in experience varies widely. Some owners report the headphones sound great out of the box and they did not hear significant differences over time. Others found harshness and restricted bottom end to be very distinct.
Most owners find that the headphones become noticeably smoother after 18 hours or to 3 or 4 days of moderate volume playing time. Most of the benefits of break-in are to be had in the first 3 weeks of possession, but reports vary.
There are however, some clearly defined effects on break-in time. The longer the speakers are played and the louder they are played will determine their break-in status. Eventually, the changes slow down and cease to be audible. Headphones are mechanical devices so not only will time and energy be factors but so will heat and humidity. If you are using an iPod, a new amplifier, CD player, or cables their performance will be changing as well.
The fastest way to break-in the headphones is to leave them on at moderately high levels. This might not be recommended for owners who use with tube amps but for conventional solid state gear, there are not likely to be heat or instability issues which will harm the amps or the headphones
You hear differently from day to day depending on atmospheric changes and the condition of your sinuses. As you become accustomed to the headphones and the system, you stop listening to them and listen through to the music.
When the time comes that you only hear music when you turn the system on, the headphones are broken in, your electronics are broken in and your ears have determined that they really do like what they are hearing.
Break in will be the same for any of our headphones but the mids will be smoother from the start due to reduced earcup modes and therefore, breaking of the mids will be more difficult to detect.