Something to remember:
A lot of "old" drivers were almost always paper coned (ditto for the dustcap) with only a handful of options for surrounds- usually treated paper. There weren't as many available materials back then, and most consumers were driven to buy when said speakers had a quick transient response (JBL's popularity was largely due to this). Paper was (and typically still is) the material of choice when transient speed is a primary design goal.
I suspect a lot of vintage headphones were made of very simple paper drivers that didn't require a lot of break in because there wasn't much going on with the material itself. I can tell you that an all paper coned driver becomes lifeless in time and requires a recone even if it still works and specs properly.
A contemporary speaker or headphone could contain drivers made of a dizzying array of materials- from alloys to pulp composites, to carbon fiber, et al. These materials are popular today because today's audiophile isn't so easily duped by transient snap alone. We want out cake (detail) and we want to eat it (smooth linear response without break-up distorion) too. New materials are complicated and proprietary and I (personally) don't think there is any BS WRT recommended break in times via manufacturer.
The suggestion with the 'techno in the basement' is exactly what I was going to say. You have to consider that ears, like eyes, are easily adapted to something new. What may intially appear as a "bright" sound will eventually trick you by virtue of the taming nature of our minds. After a while, it won't sound as bright because you get used to it (compensate for it). We have these litte internal EQ devices that lure us into bad sound. How many times have you heard a "refresher" product that woke you from the spell you were under, and upon returning home to listen to your once "great" sounding rig- was now completely horrible!?
To be more scientific- you gotta play the cans
without listening to them until a few hundred hours pass. Then put them on and give them another try. I understand your concern for a return policy, but you could be missing out on an amazing pair of cans because you weren't intially impressed. That would be a shame. A lesser evil is that, after the required break-in, you still don't like them and put them up for sale ion the used marketplace and end up losing only a few bucks.
If you don't want 'em after break-in- shoot me a PM! I am so close to hitting the trigger on a pair. A "used" pair for slightly less than retail is making me salivate!!
