celestion 7000

Hey, so I looked at the review and the close-mic measurements.  I have further thoughts.  About the best you could do is clean up the tweeter caps OR ...

a fully active conversion.  Get a 2-channel plate amp with DSP crossover.  I _think_ you could push the woofers up a little higher, to say 3 kHz, and clean up the tweeter response by a lot.  You need to measure and experiment.  Measure the output of the woofers in front of the tweeter angle to see how the arrays work off axis.

The simplest thing would be to rreplace any tweeter caps.  Leave the rest alone.

what do you need to improve?

famous for its drivers. I still listen to my Celestion 5s every day. Ridiculously great, musical, detailed sound.

Great speakers! As @stereo5 said, maybe replace the caps as they've been around for close to 30 years. Are you comfortable soldering? If so, it should be a fairly easy job to swap them out for some really good ones. Here's a 20-year old post about the "substandard" ribbon transformer and what you might do with it. Also, an equally old Audiogon discussion about them. 

An electronic repair store in your area should be able to easily perform the upgrades. 

You could possibly replace the crossover parts and wiring to more high end standards.  There are a lot of guys here that do just that, perhaps they will contribute.