Hi vadimgal,
This now helps to explain quite a lot, and I had rather guessed that this was not a question of a simple cap substitution being the cause of such dissatisfaction.
Firstly, you did a very sensible thing in retaining one speaker as original, and I always recommend this to be used as a yardstick for comparison purposes. When carrying out any x'over modifications and for whatever reason, it easy to gradually veer away from the original overall 'voicing' of the unit unwittingly, especially if this happens in small increments, say when changing one cap at a time.
Now that you have relaced the original parts in this speaker, anyway, why not try just substituting one component at a time (which again I would always suggest) and then you will get a much clearer idea of which changes affect the overall sound.
It is not easy (at least for me, anyway!) to diagnose precisely where your problem exists with absolute certainty, and there could be several reasons for your dissatisfaction. For example, you say that one cap nominally 6uF measures at 6.8uF which is quite a difference, depending to some extent where it is in the circuit, but as advised earlier, it is quite possible that KEF chose this value deliberately. If this is a film cap, I would be very surprised if the value has changed much over the years, as in my experience most film caps are extremely stable in value over time, and I have measured many caps which have been in use for 30/40 yrs. If it is an electrolytic, these do tend to vary more over time, though.
Do you have a schematic for this x'over which you can post, as I am amazed at some of the rather high values of caps which you say are in this x'over. It is unusual (for me) to see so many caps which are over 80uF and up to 1000uF in one x'over alone, but after some 40 yrs of playing around with passive x'overs, I am still learning!
Regards,