I have a pair of JBL L112s I still use as my main speakers. The foams went out about 2 years ago and I had them reconed by an authorized JBL repair shop. He did a little work on the grills and said he would check everything else and set to factory specs. They sound great! I bought this pair in 1983 while in the army in Germany. Please keep in mind this recone cost over $300.00. I was told and saw on the web JBL does not accept simple refoaming as the proper fix for deteoriating foam surrounds. I'm sure there are those who disagree with this. However, on at least two occasions I emailed ebay sellers of vintage JBL who claimed to have refoamed the surrounds instead of reconing, and asked them if simple refoaming was proper. Neither one responded. I guess you can interpret that any way you want. To answer your question, yea they can sound great after 20 years if you have someone like me taking care of them.
