Speaker Break In...? Or listener “Break In”?

The only difference I can hear consistently, every time, is how my system sounds immediately after start-up and during the first twenty minutes or so. Definitely improves during that time. Have never been able to positively, consistently hear any improvement over hours of use with any gear except my cables and ICs. I also note the system sounds different at various times of the day or night and seems to sound its best on Saturday mornings before 10 AM. 
Not taking sides in this thread, just stating what I've heard over time.
As always, YMMV.
Remember, it's just a hobby, not brain surgery.
Keep it light, keep it fun, have fun, happy listening :)

About decade ago I acquired a thirty year old set of Klipschhorns from a good friend of mine.  Previously I'd been listening to a small set of Dynaudios, which I liked very much.  Upon first listening to the Khorns they sounded harsh to the point that my ears would cringe at certain passages of some of my favorite recordings.  However over time the harshness disappeared and I was able to appreciate the dynamics they were capable of.  Since the speakers were already well broken in, I attributed the change to my ears.  One day I decided to fire up the Dynaudios again, and now they sounded muffled compared to the Klipschs.  Same amp, wires, and music.  This doesn't mean  I don't believe that  some physical break in occurs, just that our minds can play tricks.  Many years ago a piano tuner advised me, "If you keep playing on an out of tuned piano, over time your ears will adjust to that out of tuneness so that when you play on a properly tuned piano, it will not sound right".

Why do some driver manufacturers recommend a "burn in" period. Say, 20 hours continuous music a low level?  Varnish on the cone? 
My own experience in 30 years of working on professional broadcasting equipment, is that the ear adapts to accommodate different loudspeakers. 

Somenone says the sound improved after two hours I might believe. Someone says the sound improved after 400 hours I wouldn't. That's just sales talk.

So annoying to hear categorical claims such as 'all do' or 'all don't.'  Does it not occur to the people making such claims that 'some may' and  'some may not?'  Like many claims of physical characteristics in audio, these should be amenable to physical measurements. I spent 5 years as the "human factors" postdoc in a department of electrical engineering and saw this being done all the time. Are there no real engineers in audio?

Notice I don't say measurements that people can detect any difference.  That type of measurement is much more difficult to carry out given the inherently noisy nature of human testing.