Thoughts on extending speaker life.

Hi,
for rubber surround i use talc powder application every 2-3 years and so far they behave like new. Like giving spinach to popeye. Do not touch rubber surrounds with your fingers. I would avoid petroleum based products, they don’t smell nice. My Dunlavys sound and perform the same after so many years.

Get a great speaker like MAGICO!!

I would not hesitate to use armor all if i thought the surrounds needed it.  i have some original polk monitors that are over 30 years old and show no signs of deterioration- but they are rubber surrounds and the grilles have always been in place.   foam surrounds will just need to be replaced if necessary.  not that difficult.   
don't sweat it.   

Age can deteriorate any part, cone, surround, you could burn a voice coil, ....

I have speakers with drivers made in 1956. The tweeter horns and mid-range horns appear to be bulletproof, 63 years old, still sound terrific. I do have spare drivers waiting (as someone else said, they were made in 1956 also, they just haven't done any work).

The 15" woofers (paper with flexible pleated cloth surrounds), this one

https://reverb.com/item/2307415-electro-voice-model-15w-alnico-magnet-15-woofer-new-in-original-box-nos

I had them re-coned professionally many years ago, have a spare waiting, and re-coned one myself not long ago.

Videos show how to do it, cost is minimal, the hardest part is scraping the old glue off the spider ring.

http://www.speakerworks.com/speaker-repair-kits-s/65.htm?gclid=Cj0KCQjw3uboBRDCARIsAO2XcYCQUY1lUVYTubmOssOLwtflmQlsxhhxHHnNrms9nKalZrIaNGzk2ngaAqehEALw_wcB

Based on the success, I re-coned my Velodyne 12" sub, it went well also.

I used to think about 'sag', especially such advanced years, but there is no evidence in mine. The voice coil would rub, you would hear it for sure.

I should have said, 

15" I replaced the coil, cone, surround

12" I just replaced the surround, the cone and coil were fine, and getting the voice coil centered was easy after watching the video.