JBL L-300 Summit Are they still any good?

Actually, there is a lot you can do with the L300. They can be restored to sound like they did when new. The diaphragms in the midrange and tweeter can be replaced if necessary, although the tweeter diaphragms are no longer manufactured by JBL. Radian manufactures an excellent replacement for the mids. The tweeter phram may take some time to source, as most folks don't recommend the Chicom replacements. Nelson Pass designed a replacement crossover that eliminates the inductor/autoformers. Search for his article on the internet.  

Thanks for the information. Do you know how I can find the new replacement diaphragms ?

Great, I love the older JBL's. I have a couple pair myself. Glad you know what you are doing. For me, Clarity or Mundorf caps, a source  Capacitors: Madisound Speaker Components (madisoundspeakerstore.com). Not sure how their prices compare. New diaphragms can be found on ebay if you are persistent. I would NOT change diaphragms unless they are really needed. Originals are the best unless they are auditorily defective. Watch out for made in China copies, they are not good.

If the 077 prism tweeters have the blue ring diaphragms, they sound better than the newer gold ring diaphragms and you simply cannot find the blue ring diaphragms as replacements. I've been watching for 10+ years and only saw one on ebay and missed it. I have one in case of a failure!

Thanks for your information pulled the other speaker apart today. Just fired them up! For old cap’s they still sound good. I have done a lot of business with Madisound with all my L-100 restorations. I still have a ton of the LE 5-2, mid
LE 25-1 tweets and 8 123A-1 and -3 woofers. I need to start unloading the extra parts. I watched a interview with Nelson Pass he said he may have improved the L 300 crossovers a bit but still felt  JBL did an excellent job on the configuration.