Can speakers sound worse during break-in period?

"break in cant be measured its not real"
This statement is a load of crap.  I've been breaking in 4  - 15 inch woofers for about a week.  During that week,  FS has dropped around 3hz, QTS has dropped about .5 which is sizable. Not sure why someone would make such a statement, many other changes,  but break in is clearly measurable and over this time drivers,  all size drivers will make very noticeable changes. Normally the biggest change comes at around 4 hours of running them at decent levels,  it changes little by little until you hear real differences again at around 50 hours and still change on most drivers through 100 hours and I've seen a few speaker take even more. 

Some things you can’t measure: sweetness, presence, fullness, soundstage dimensionality, brittleness of high frequencies, boomy-ness, thinness, air, wetness, slam, rhythm, wimpy-ness and realism.

@geoffkait  
I would argue that many,  if not most of those things mentioned also change during break in. Of course,  that may or may not be the intent of your post. 

The answer to your question is YES.  Depending on the speaker they can go through a number of changes during the break in period.