Do you think driver “break in” is real?

Do you think “high end” drivers and crossovers typically need a “break in” period before they sound their best?  I ask because, I believe I’ve experienced this first hand in a very significant way. I replaced the tweeters (same exact brand and model as original) in my ACI Sapphire 25 year anniversary edition speakers and for the first week I thought I’d lost my all time favorite speakers. I was depressed!  So I just kept playing them…  finally after a couple weeks, I’m literally stunned and blown away at how incredible these speakers sound. Completely different than the first 30 or 40 hours after I put in the new Scan Speak tweeters. So I say break in period must be real - at least for some drivers. Has anyone else experienced this to a degree that is unmistakeable?  
Crossovers too?  I just rebuilt the crossovers for a pair of ACI Sapphire XL’s (using highest quality components- same values) and so far they sound mediocre. Hoping for the same result as my other Sapphires but after about 20 hours - no noticeable difference - and they do not even sound as good as prior to crossover rebuild. They sound flat, too bright with poor imaging - but for 10 to 15 years they were pretty great sounding speakers. Thoughts on crossover break in?? 
Thanks to anyone who responds!!  


Yes of course speakers need burn in time. One hi fi dealer advised me to use burn in cd from Purist audio design and it really helps to reduce the time.

@erik_squires - indeed, once broken in, a speaker should perform how it was designed to perform and no more playing will increase the performance.

I dunno about that 'very quickly' part, though - that's not been my experience with either speakers or headphones, but I guess it depends on how you define 'very quickly'. I give 'em 150 - 200 hours before I judge them, though some need less.

My loudspeakers required about 300 hours of use to fully impart their coherence.  

For some woofers yes, they do loosen up a little. For planar magnetics and ESLs most definitely. Their diaphragms are tensioned with a heat gun they then relax to the right tension. Some ribbon speakers will actually relax to much in time and require ribbon replacement. 

@erik_squires  Exactly. Within minutes. Certainly not the "break-in" that people talk about or need to worry about. Several custom speaker builders and kit makers have run this test on drivers so they could get accurate "broken in" specs. I read the first one about 20 years ago. I believe it might have been a kit maker whose name escapes me, but it began with a 'Z'. I believe they shut down years ago. Another who did the measurements was Bill Fitzmaurice.​​