Don't push an opinion on the OP like it's a fact. There is no proof for or against burn in. Pushing your belief on someone just isn't right.
I'll disagree with BA burn in here as well as my HF2s, along with another HF3 (ABXed against an HF5 by another member) also burned in.
@OP: about burn in; basically, it's the belief that as a driver ages, it is able to break in. This break in essentially results in a change in perceived sound. Although the change is positive most of the time, it isn't always (PUR1000, Hephaes, ASG-1). The causes of break in aren't yet know to why they exactly occur. There are many theories behind why burn in occurs:
- Driver burn in: as drivers age, the sound of the driver changes due to physical changes the driver undergoes. Think about a shoe breaking in (especially if you run :p).
- Brain burn in: the idea that it's your brain adjusting to the sound of the driver with time. So as you listen to the headphone more, your brain adjusts to the sound and changes it for you.
- Tip/Pad burn in: the idea that as time goes, the tips of the IEM (along with pads if it's a headphone) will break in and conform more to your ear. This causes a better seal, but a better seal changes the sound.
- Placebo: this is the idea that the person is hearing things, but really isn't. It is normally used by people who don't hear the effect as a means to explain why they don't, and others do.
There really is no solid evidence for or against burn in. Although many here will soon start quoting Tyll's graphs, even the small change in those can be audible (human ear is quite sensitive). The question really becomes what is audible and what isn't (in the changes of the graphs over time; Tyll is not the only one to test this burn in). I'm not going to go into specifics about that now.
As for if you need burn in. If you like the headphones as they are now, you don't need to burn them in. Enjoy them and enjoy the roller coaster ride they put you through as they burn in, but there is no guarantee that you'll hear it though (as stated before, not everyone does; it's not a problem if you don't). If you do still find the need to burn in a pair of headphones, plug them into a device, play it for hours on end until you feel they are done burning in (normally around 100-200 hours; some as low as 50 though).
I didnt state burn in as fact or fiction, I simply stated what burn in is for people that believe in it.