After much listening I took time out to re assess the whole project.

Seemed to me with Tri-Amping and changing the Tweeter to the Seas unit, I was getting further and further from what was special about these speakers so I put the original HF2000 tweeters back in. After taking the 66's to a local bake off, the consensus of opinion was that the mid range needed bringing forward as it was not strong enough. I swapped the MF500 drivers for the later MD500's which I already had, as I knew these were more forward sounding. I changed the x-over values to suit both these driver changes as shown above and this did the trick. Without doubt the best they have sounded. I used a mixture of caps for the crucial 24uf in the mid range filter, approx 95% Mundorf ECap (Plain) Bipolar electrolytic and 5% Ansar Supersound Polyprop. This produced the right balance, the PP adding a bit of extra body without the problems of using a total value of Polyprop. I'm very happy with these now and the circuit/parts/layout I settled on can be seen in the PDF file linked at 66_13.

Final Version 

So after all the experimenting I have ended up with capacitor values exactly as the original ones that Celestion used and they have been replaced like for like, that is to say, film caps replaced by film caps and electrolytics replaced with electrolytics. This sounds excellent and better than all the other versions I tried. Do not be tempted to replace everything with polyprops, it just does not work and in my case sounded worse than the old Elcaps.

The Brand of film cap you use in the HF filter is important and I found Ansar Supersound worked best over all. The critical 24uf series cap in the mid range filter is also noticeably effected by the brand used, Mundorf ECap worked best with about 5% Ansar Polyprop in the mix as exsplained above.

That's me done on this project and I will now enjoy listening to these very nice speakers.