Have speakers really improved within the last 20 years??

There is no one single measure that I know of and I both like and hate the, 'how does it sound to you' answers.  The huge difference over the last 20 years is found in 'material science'.  The raw materials have developed in all aspects of audio into new 'materials' and this is true from the material for cones of drivers to the construction of electronic components and the qualtiy of everything from contemporary Cardas copper wire to silver lined capacitors, etc.  The new materials allow for higher SPL/sensitivity without loss, and in fact performs much better than weak/low spl speakers.  If you take apart a common 1970s speaker you will find lots of really crappy junk stuffed into them:  Sony, Bose, Sansui, Henry Kloss's ARs and Advents, and even Magnavox et al.  
If you start with speakers from the 60s and 70s you are able to track the change in design and materials better and able to see the trends that began in the 60s and are still 'trending' today.  And it also depends on which speakers you are talking about.  Kenjit describes a cheap commercial speaker and is accurate.  If he took a close look at a pair of $140,000 Focal speakers it does not look at all like his description.  There is alot of room inbetween!!! 

My understanding is that desktop engineering programs have improved significantly. Engineers can work through design iterations much faster. Reflections, resonances, stress, etc can be analyzed quickly and accurately. This has led to better designs across the board. 

Use your ears. If you don't hear a difference between speakers manufactured decades ago and some of what's available now, count your blessings. You'll save money.

The answer unfortunately is no. 

By the late1950s the main loudspeaker technologies had all been invented.

Sealed box, open baffles, horns and even transmission lines were all in existence by then.

Dynamic drivers, dual concentrics, paper/ polypropylene/metal cones have long been in use. Electrostatics, ribbon tweeters etc have also been in use for decades.

So what about the improvements? 

I think, if there are any, they've mainly been centred around tiny incremental engineering improvements and the use of software to juggle and hopefully hide the various performance compromises that all loudspeakers by necessity are subjected to.

You'd like to think all modern speakers must measure better but there's little evidence to support that.

The new KEF LS50 Metas are still a dynamic driver in a box, but a highly refined one that offers a performance never previously available at that price point.

Are they a huge step forward or a small incremental one? I don't know, but the speaker I'd want to compare them to would be the LS 3/5a, a renowned long time standard.

However I am willing to bet that the KEF is the far cheaper to buy if you were to adjust the BBC design for inflation.

Twenty years is too short time period. Question should be thirty or forty years. My most memorable sound experience comes from stacked Quads 63’s with Kef black boxes passive subs in the late 70’s. Playing Little Feat Columbus live. And miss my Beveridge towers...