Do you guys always replace diaphragms as a pair?
For me, the exception to "always" is in the event that a fairly new one, or one with known limited use is inadvertently damaged/destroyed by known human error.

I'm totally of the school of thought that the surrounds fatigue, certainly the old style lead in conductors did/do, and i'm sure the surround area does too. Logic suggests that this would change the parameters to a degree as well.

Maybe it's just placebo effect, but i swear that to me new phragms sound a bit crisp on top and mildly constrained especially down at a 500hz XO until they have several hours play time.

While there's some different forces at work, i'm reminded of the guitarists grin as he plays for the first time after tuning up a new set of strings.