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Three cheers to Jazz...not only because he has my son's cool name as a moniker, but, because, besides giving his humble opinion, he has done something to try to prove/disprove the question at hand. We are all supposed to be talking about the question at hand started by this thread, right?

Although Mike's posts make for rather interesting and "moth to the flame" reading, I can't help feeling as if I am watching a police officer or court judge, cite the need for burden of proof. I have learned so much from this forum, and it has truly enriched the musical part of my life, but I feel Mike's posts are more for entertainment than purpose. As a psychiatrist would say "I sense some anger underlying in your statements". Life is to short to be so aggressive on a simple question to the group.

JaZZ: Interesting post. You have any idea what the cones on those speakers are made out of? It would be interesting to see a graph of the resonance change with time.

I made an observation that may be of interest regarding burn-in. When I broke in HD650, I used bass tones for the first time and got an impression that this helped or speeded up. I had another pair of headphones that I considered burned-in, new but burned-in 100 hours + some listening. I decided to try if additional burn-in with bass tones had an effect. Judgements about this is of course problematic considering the observation and reliability problems that are associated with these sequences where you can't move back to the prior state and compare. One can choose not to test for this reason, but my goal was not a scientific inquiry but a possible improvement. It can also be regarded as stupid not to try because you lack scientific evidence and miss possible improvements.
So I run some bass tones on the phones. I started with a 200 Hz tone to set the volume and then heard an audible distortion (400, 600 etc tones). Nothing remarkable because the volume was high. The thing that surprised my was that the distortion disappearedafter some hours of bass burn-in (16 - 200 Hz) at high volume. I have repeated it on another pair (another model) and think that the distortion decreased but it was still audible.

Daffy...
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You have any idea what the cones on those speakers are made out of? It would be interesting to see a graph of the resonance change with time.

...the cones are made of kevlar. But they have nothing to do with the resonance-frequency change -- that's in the sole responsibility of the suspension components, thus rubber surround and spider. Both have obviously changed their elasticity modulus by applying mechanical stress over a certain period, and permanently so: this can be claimed with certainty, because the new values are much closer to the manufacturer's specs.

Unfortunately I can't show you the graphs you're looking for; I have no frequency-response plotter. The measurements were taken by hand, using a tone generator and a voltmeter.


Anders...
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...I started with a 200 Hz tone to set the volume and then heard an audible distortion (400, 600 etc tones). Nothing remarkable because the volume was high. The thing that surprised my was that the distortion disappeared after some hours of bass burn-in (16 - 200 Hz) at high volume.

...that's interesting! I've never encountered such distortion phenomena (maybe some rattling here and there that disappeared after a bouncing hit), but I don't doubt them. One can only speculate about the reason. Maybe it's even the same as the one offered by Stax for their electrostats' break-in scenario: an equalization of some inhomogeneities with the mechanical tension over the entire membrane surface (hand in hand with a loosening, of course), considering that headphone membranes represent membrane and suspension at the same time and are indeed sort of clamped on their baffles...

JaZZ, it can depend on the type of headphone driver. The first headphone was a Grado RS-1 and the second an AT W11R. You must also be a little careful with these because the drivers seems to be a little weak when exposed to large bass volume. I set a high volume on the Grado at 200 Hz and noticed a clicking when the disc moved over to tracks with lower frequencies, and had to lower the volume somewhat. The Grado seems to have some type of max bass output, you can increase the volume to some unlistenable level but the bass becomes not stronger after a while.
The Sennheiser HD650 seem to be a more robust constructions in this aspect and had hardly any audible distortion. Distortion is easy to hear with sinus tones.
I think the Grado gained somewhat in clarity after the bass burn-in. So my current hypothesis is that at least some headphones need a heavy bass burn-in for maximum performance.

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interesting result but again this is only conculsive if they had the same sound out the box ( many people say that there is often a significant difference from identical headphones out of the box - see an illustration of this measured in grahic7 : Speaker Break In: Fact or Fiction? - page 2 — Audioholics Home Theater Reviews and News ).

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here is one expriment that measured no break-in : Speaker Break In: Fact or Fiction? — Audioholics Home Theater Reviews and News
and another one that that measured some break in : http://www.head-fi.org/forums/593776-post45.html

I personally thought the idea of burn in was ridiculous, I listened to my IE7 with some classic rock for what seemed like forever. I decided to try some rap music and the bass seeped all the way into the treble. No exaggeration I could not make out the words. I let them run on some bass heavy tracks to let them "burn-in", the entire time I was laughing at the idea that it would actually work. Bass transparency improved, a ton. So much it was impossible for it to be placebo because I listened to the same song, You're a Jerk. And I could understand the words with the bass bumping away.
After this I decided to burn in my Skullcandy 50/50 and my iBuds. The Indus had no change and the Skullcandys had such little change it was probably placebo.
 
Headphones are like BASEBALL MITTS. They all need to be broken in but some only take a little bit, like cheap rubber mitts, while others take a long a** time, like leather mitts. I find generally speaking that those who do not accept burn in tens to be overall skeptics and less experienced with headphones than their burn in friendly counterparts. By no means am I trying to insult anyone or call myself an expert but I just thought I'd like to give my unbiased opinion as I was swayed from one side to the other.