I hate to stir the pot, but wanted to add one more data point for people.
As a matter of personal opinion the HE-500 does have burn in. And 150 hours is just about right. I always said 120 minimum and after 200 no change ("stable")
This coming from someone who has his 4th pair of HE-500 and burning them in for the 4th time. I'm a big believer of the scientific method and a proud skeptic. But not everything shows up in Frequency Response Cuves/CSD/Impulse Response/Square Waves.
I think we need more sophisticated measuring tools and then we'll be able to measure small differences. Like being able to measure the diaphram with laser precision while real music is playing, instead of sending impulses to make measurements.
Now... saying all that can it just be a placebo psuedo effect and my brain is being fooled for the 4th time? Of course. Humans are faulty in general so why not.
I hope in the future we'll develope more sophisticated measuring tools to really flesh this issue out.