Break in is real. If you have the equipment to measure frequency response you can see it for yourself. Check the websites or manuals for any decent speaker manufacturer and you will see a mention of break in or run in. You may or may not hear the difference, but it can definitely be measured. In my experience the sound difference is most noticeable with larger speakers, but even some headphones sound better after their break in period.

https://www.klipsch.com/blog/how-to-break-in-a-speaker https://www.eminence.com/speaker-break-in/ https://www.sweetwater.com/insync/how-do-i-break-in-a-speaker/

Ok tired of looking for sources, but I hope you get the point.