Buying used vs new speakers from a technology perspective

OP is your question answered, or are you still unclear?  If so, what?

Pretty thoroughly answered yes. Many thoughtful answers from all sides. This will help me when purchasing new and/or used speakers in the upcoming future. Much appreciated guys! 

its all in the drivers working with each other   what freq's  sounds best from the drivers you use and how loud each driver is producing its best sound no over laping freq's as this is the cause of unclean sound    paper cones can degrade with time moisture and how hard they've been played  but possibly still sound great 

There has been really no improvement in the overall technology of speaker design in decades. What has been improved is the reliability and durability of the components used to make top quality speakers. As an example Quads have been around since 1956 and people loved them but they were notoriously unreliable and fragile. In the late 70s Acoustat developed the first bullet proof electrostatic speaker. You have to attack the speaker with a spear to damage it. Sound Labs took the concept even farther and they have been at their current performance level for some 20 years. 

For the past 15 years I’ve had a series of 2-way speakers in my 2.1 home office system, which I’ve continously upgraded.

Here’s a factor no one has mentioned: it may be harder to find among modern designs, a speaker configuration that was more commonly used in vintage speakers. In my case it’s sealed/acoustic suspension design. That sounds best by far in my limited-space system, and it’s much harder to find now than in the past.

I’m currently using a 35 year old pair of KEF 103.2 speakers: 2-way sealed passives with 8" woofer (bextrene surround, not susceptible to rot) and a silk dome tweeter. The previous owner replaced various capacitors in the crossover and bypassed the limiter circuit, which has more capacitors. These speakers are the best I’ve ever heard in my system (the 6th or 7th pair I’ve had).

In my experience, there are no hard and fast generalities about vintage vs new gear. When it comes to sound, trust what your ears tell you...

Over the past 20 years or so I feel a big part of it is voicing. Went from super fast, detailed, analytical type speakers from the early 2000s to the past 5 years or so coming to tone, warmth etc. Obviously there are brands that have kept their same schtick forever but I’m talking about as a whole. 
 

As far as actual holy moly improvements, I don’t feel as there has been any major improvements. I do love me a well implemented BE tweeter though. 

I still enjoy my 1940s RCA photophone horns the drivers are 80 years old or so they are still in spec and sound amazing. Even my jaded audio designing friends have had to rethink what was possible then and if we have made that much progress since.