Sometimes dust and animal hair can enter the Quad ESL 63 and cause arcing. I suggest disconnecting the speaker, letting it discharge for a half hour, then rapping the sides forcefully. I have had some success with this method over the years.
Let us know if this does not cure the popping. If the popping is disrupting your listening, or non-listening sessions, you may be discovering that it is time for a panel replacement, or complete rebuilding (not a bad idea if you want these things to continue to make sweet music for another 20 years or so).
for the music,