Speakers and headphones can sometimes loosen up with use at first. Most models don't to any audible degree; if they do, the result is slight, not "night and day."
Our hearing and our perception do have large, noticeable and unpredictable vaiations. These are usually confused with changes in the equipment. Experience, non-newbie status, does not always help. Skepticism about the cause does.
I have been at this for decades and two years ago I changed a preamp from a normal unit to a no gain buffer unit, basically eliminating a piece of the usual electronics we all use. I thought it sounded terrible, but we do all seem to have our favorite distortions and five decades of listening did not make me immune. After using more accurate gear for some time I tried it again and I recognized it as a minor but significant improvement. As usual, it was my perceptions that needed breaking in, not the gear. Call it getting used to the new sound, if you like.
Nothing else beside transducers that move break in - electronics, wire, capacitors, amps, players, DACs - nothing. A few types of electronic components do change VERY gradually over time with use, tubes and large electrolytic power supply capacitors come to mind, but they are heading out of spec, not improving. Tubes can last for years of use and the big electrolytics can take twenty years to dry out, so don't imagine you can hear these things during a break in period.
Edited by Clarkmc2 - 10/21/12 at 8:02pm