Have speakers really improved within the last 20 years??

Question:

• If there is one measurement that would prove that speakers have indeed got better over the last 20 years, what would it be? 

I dont just want one example of a speaker from today that has a better measurement than another speaker from 20 years ago because that could just be a coincidence. I want to see IRREFUTABLE PROOF that most speakers today have a measured performance in at least one area that is better than most speakers 20 years ago.

When I look at a typical bookshelf speaker from 20 years ago versus one from today i see little difference. All i see is a wooden box, typically mdf with a pair of drivers in 'em. There would be a small crossover circuit inside and a bit of foam inside the box and that would be the end of the story. I would like to believe that speakers have gotten better but I see no reason to believe it. All I see is that speakers may have gotten brighter and brighter with time to dupe us into thinking we are hearing more detail. 

This challenge is open to any audiophile or speaker designer reading this.