Can speakers sound worse during break-in period?

It's been my experience different brands break in at different rates - simply depending on the materials involved. Revel says no break in time required, Buchardt at least 100 hours.
My Thiels probably took 2-3 months to break in, and it was quite noticeable when they did. Bass isn't something you imagine. The midrange also lost some forwardness/harshness, and people who'd heard early then later concurred. However my Maggies sounded great from the start and changed very little other than a bit more slam. Think ribbons are good to go out of the box.

I'd imagine different drivers are going to go through changes at different rates, so yes I can believe it could sound worse for a bit until they all get in sync. What I plan to do when I get my next set is simply put on the Stereophile test CD, and run it 24/7, louder when I leave the house.

If you can't measure it it isn't real. infantile.

"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.