Well, the mystery of the underperforming KLF-20s is solved, thanks to Bob Crites and Carl Richards in Connecticut. I'll try to keep this brief but detailed enough for someone with similar issues to have a corrective action plan in the future.
Problem: Woofers occasionally crackle and pop during some bass passages. Overall speaker performance is sub par, muddy. Speakers are being driven by good quality gear, Sansui 9090, G-7500, Marantz 2250.
Corrective steps: Removed all speakers and drove them down to Carl Richardson at Carl's Custom Loudspeakers in Hartford, CT. He tested all the speakers via a low powered frequency response test and all speakers passed with no issues. Computer printouts showed good linearity and very good performance within all the speakers useful frequency ranges.He also checked for rough spots in the coil, fractured spider, etc. All were good.
I conferred in parallel with Bob Crites and discussed this problem. He suggested a crossover rebuild as a first step. I sent him the crossovers and also ordered the Ti dome replacements for the tweeters as an upgrade, not to resolve any problems.
I also purchased some basic 2" polyurethane foam and ran it along the back and sides inside the cabinets.
Solution: Received the rebuilt crossovers and performed the tweeter dome rebuild. Wired up everything and whoa........... my speakers are back!
The accuracy and presence is unbelievable. No pops and the foam has quieted the "boominess" inherent in these speakers. There's still plenty of bass, but it's more natural now. One other thing, I hardly need to use any power from the recievers, at around 10 o'clock, you can't stay in the room it is so loud, but uncluttered, undistorted loud. I am very happy with these results.
Summary: Mr. Crites is top notch. I asked a million quesitons and he patiently answered them. The results exceed expectations. Hopefully I can now enjoy these speakers for another 12 years! Thanks to all for your help on this problem! Dan