If flat is your only measure, you can stop at 1980s monitors, be they JBL or Yamaha, ... flat is flat no matter which side of the country builds them. I know, the "West Coast" vs "New England", install and measure a few thousand pair and I"m sure you'll find that flat is flat.
Imaging, time alignment, phase coherance, cone breakup modes, none of these things were discovered nor invented post-'80s, I was designing electronic circuits on CAD in the '80s as were my coworkers designing mechanical assemblies. There were modal analyses, FEA, analysis of everything from wave propogation to strength of materials and natural frequency. And it was accurate.
I'm sure that global climate change has had a profoud effect on how sound is produced and how it travels over the past 30 years and has changed how we need to design speakers, or not.
Ears have not changed, air has not, although speaker technology has evolved. Pulp speakers with aquaplas with edge-wound ribbon coils has changed to pulp speakers with aquaplas and edge-wound ribbon, aluminum and titanium diaphragms with foam damping material have changed to aluminum and titanium diaphragms, some with aquaplas-type damping, film caps have changed to film caps, and copper wound inductors have changed to copper wound inductors. Vast and sweeping changes in materials.
There were carbon-fiber cones, poly cones, reinforced poly cones, ceramic-composite cones, all in the '80s I believe as I have (or have had) examples of each. Satellite-subwoofer systems existed, good ones. If you get a chance to talk to some of the great speaker Designers from the '90s I'd like to be a fly on the wall, hearing some of the great speaker critics of this forum quickly reach the limits of their knowledge and experience. None of this stuff was unknown when they designed speakers.
One thing that has changed since the '80s is the ability for a homeowner to buy and operate measurement and adjustment devices. One can now buy a calibrated mic, software/freeware, and a parametric/digital equalization that allows us to adjust any response to completely flat and even correct phase-distortion (but not doppler-based) in our audio systems.
Another thing that has changed substantially is music. In the '80s we started to see digital-sourced music, that 90dB dynamic range that became available on the CD format, with response frequencies down near DC/Zero. That taxed audio systems to create huge powerful dynamic peaks that previously were only in live music. I remember Flimm and the BBs mastering on digital media, which became the every booth at CES. A lower power system or cheap speakers would completely choke trying to reproduce some of that stuff, ... but when it played well it was amazing to hear and feel. Electronic music has gone a long way from Freebird or Frankenstein to Deadmau5 and others, playing BT to demonstrate in 2000 was as impactful as anything prior (and one can now actually come close to the Telarc 1812 Overture cannon blasts).
Regardless, painting vintage vs new as an evolution is accurate. Using the same broad brush to say that there is nothing from the '90s and before that can produce exquisite sound quality IMO is not accurate. Being specific and saying that "AR3 are not able to properly reproduce xyz music because it: isn't able to produce the dynamic range, has too much phase-shift at crossover frequencies, doesn't go low/high enough, isn't flat enough, ..." might be accurate (just a hypothetical example, no AR3 owners were harmed). OTOH, playing 1970s music on the same speakers might be reproduced just as the artist and Mixing Engineer intended it and there isn't program material on the master that the speaker can't reproduce.
Enjoy whatever speakers you have, modern and vintage, there are good and bad examples of each. In engineering there is always the theoretically perfect, and what actually works. Many of the modern designs that are built after the designer has read a few books and taken some classes but doesn't have a lifetime of real-world experience are only good on paper, and can sound pretty much like crap. I used to be that spec-chasing guy listening to the equipment instead of the music also, ... we get over it.
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