Sound quality of Newer versus Older speakers

My old BIC VENTURI - v-830 and - v630 are still awesome!

    Some of the best speakers I bought. Still in use. Cerwin Vega D-9’s still in use. 

Materials get better, componentry gets better, boxes get better, drive units get better.
One of the major differences is the HF extension.

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The one thing most are missing here is crossover parts quality. Mundorf and Dueland have turned the capacitor industry on their heads. Even Wilson is making their own caps now.

I changed out the Mundorf EVO caps on my Nola Viper 1AX's tweeters/mids to Mundorf Silver/Gold oil and the difference was NOT subtle and that's going from a very good midline cap to a top of the line caps. Resistors also fall in that line as well as inductors but caps are the most notable. 

In modern speakers, in general, tweeters are getting better. But mid and bass drivers are getting worse.
But there are number of exceptions. JBL produced excellent tweeters from end of 60x.
Dick Sequerra vintage ribbon tweeters (Pyramid T1, T9) are very good.
Modern full-range drivers like AER, Voxative, Fostex are very good.

But in general modern bass-midrange drivers trend is cost reduction! They can tell you about "state of the art" but it is lie!
All these low sensitive tower speakers with small, dull, reinforced concrete drivers don’t have any definition from mid-bass to upper midrange compared, for example, to these Jensen from 1920’s.
They also sound compressive and tiresome even is your power amp have a power of nuclear power plant.

The good new we can mix old and new technologies today. We can use best old and new drivers and modern crossover parts.

If you google the white paper for the Thiel CS6 speaker, you will find that Jim Thiel was addressing time and phase coherence and energy storage decades ago. All of the things that have been mentioned here is not new. I’ve had my CS6 speakers for 16 years now. They still thrill me, especially with my new monos and digital front end (vinyl is still king). Occasionally I get to hear some newer speakers but nothing good enough to drive me to change. The new Sasha DAW sounds good- nice bass. Still, I just can’t see them moving into my listening room. Oh, if they had thrilled me I would have tapped my 401 for them. I would like to hear the Magico’s sometime. 6 hour drive for me but why not? I’m retired and I got my 2nd shot yesterday.