JBL L-300 Summit Are they still any good?

Clean them up and turn them up! 

I definitely will once I get the other L pads cleaned up.
great care is being taken not to damage the consecutive serial number foil cals. With all the L 100s I’ve restored I definitely have it down to a science.

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.