JBL L-300 Summit Are they still any good?

Why do you want to change the L-pads, are they bad? Just remove the assembly from the cabinets and clean/lubricate them well. I doubt you'll find L-pads nowadays like the quality of the originals. If you're not going to bi-amp with an active crossover, I would replace the capacitors.

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!

Yes JBL was one of the top back then, older ones, 60/70. I'm sure the caps are still good enough. As you know, they have made some improvements in capacitors since these were made. Could have already been upgraded I suppose.