My input on a place to start would be to look at current manufacturers that were around 20 years ago, survived and are well-regarded today. If they survived and succeeded over the last 20 yrs., seems good odds they'll be around another 20. Opinions might vary, but I think McIntosh, Audio Research Corp. & Pass Labs should be on the list. There are no doubt others.
What audio components are heirloom quality?
I am trying to think 20 years ahead but have a sincere desire to put an audio system together with the intention of one day passing it on to my child. What do you consider to be heirloom quality? What is the audio version of the 67 Shelby GT500 that will help keep the next generation engaged in the hobby and nostalgic? Speakers?Amps? What would you want to pass on to your child 20 years from now?