Questions About Speakers

1. Not really, speakers tend to last very long and look at the huge market for vintage speakers.

2. No. Everyone has their own taste and plenty of great designs out there using paper woofers and silk domes and wood boxes.

3. SF post FS the speakers got a bit more modern sounding aka bright and lost some but not all of their musicality.

I can't really answer the rest of your questions other than to say your Dartzeel amp should drive a pretty wide range of speakers. And matching speakers to room really depends on the specific room and speakers and also your expectations.

Every speaker and situation is unique so you really can’t draw absolute conclusions. Everything depends.

1. Speakers with foam surrounds will typically rot in 20-25 years and need to be refoamed or replaced, varying with climate....rubber surrounds on the other hand can last much longer....i know of several that are 40 years old + and are are working great.  It’s not a bad idea to rotate woofers 180 degrees to offset any gravitational drag on the suspension, but I have some that I’ve never rotated that are 35+ years old with no issue. If a tweeter has ferrofluid, it’s an excellent idea to clean out the gap and replace with fresh. Capacitors can get old and values can drift, but it again depends....electrolytics are more likely to drift than good polypropylene. If not abused, there are speakers that can last 50+ years....perhaps lots more.  Most resistors are fine from an age standpoint, but many resistors have an inductive byproduct that can degrade sound somewhat...I'd be wanting to replace any inductive old style style sand cast resistors with wire wound non-inductive types for best performance.  Inductors aren't likely to age, but if iron core inductors are in the mids and treble, those could be improved upon by replacing with better air core inductors.  Just be sure the inductive and resistance values are the same.  

2. Materials and technology have improved, but not by leaps and bounds, and many companies don’t actually utilize the very best available because it tends to cost a fortune. The laws of physics are unchanged. Great speakers from 30+ years ago, can still be amazing today. The marketplace has also changed. What used to be small companies with excellent sounding speakers, often change ownership, get bought, or grow to big corporations that answer to share holders and a board of directors. In which case, profit is a higher priority than furthering speaker technology.

Speakers are so subjective, plus quite room, and even amp dependent, that in the end you really just need to please yourself. Room dimensions/shape can be as important, if not more, than room size, but one size does not fit all.... odds are good you’re situation is unique. Speaker selection will have to be done with trial and error, but if the room is huge, larger woofers in some form or another make sense, If the space is small, don’t overwhelm it. What sounds best to you in your room with your gear is the speaker to get, Buy what you love.

I agree with the comments.  I’ve had speakers last a long time but not forever.  I have had a bad experience with high-cost powered speakers not lasting as long and needing repair or replacement.  I’ve had passive speakers last for 20+ years with no significant maintenance or issues.  I say if you are happy with the sound keep them for as long as you can.  I have no experience with the specific speakers you asked about.  Enjoy! 

I recently gave my brother a pair of Sonus Faber Concerto from around 2000.   Still look and sound great.   

A lot has to do with the inherent quality of parts and materials used and the climate/ environment they are in , but speakers can last a long time 

Oh  boy, here we go. You'll benefit much more greatly with speaker placement and/or some room treatment. Cables can't correct those issues no matter how much $$ you spend. IMHO, they can become expensive tone controls. That's just me and my experience but my system may not be (nor my ears) resolving enough to detect these minute differences. That being said, have some fun!