My observations are the opposite actually. New speakers always sound louder to me at the same preamp setting.
Some new speakers can at first sound like they have more bass because new woofers tend to have more overhang than broken in drivers. It doesn't mean they're more sensitive though. They just load the area with resonance at certain frequency bands. Same can be true of tweeters that "ring" when new but smoothen up with time. It is a frequency related boost and not a nominal one. As we all know, lower impedance speakers play louder but since the question is sensitivity this is at the expense of a greater load on the amplifier. We also know that "plays louder" is very different from being able to play loud with a sense of ease.
I don't think the sensitivity increases but that what is perceived is an increased sense of ease because compression at the driver limits decreases as they reach steady state. That's quite normal for most any speaker with dynamic drivers with surrounds that need flexing Gavin
Published ratings are made with fully broken in speakers, if at all it is the driver impedance that might be up a bit from published specs when new.
Okay. Now this sounds really confusing. As a speaker breaks in the nasty boom and sizzle go down but the low level resolution increases. The net effect could be either a downturn if the owner liked the boom and sizzle in the first place or an upturn because to get midrange clarity and definition he doesn't have to play his system as loud as he used to.
In the end I think this points to how inadequate the present standards for measurement are for folks like us who take tis stuff more seriously.
