The circuit design used in the 66 is electrically resonant and the damping factor of the electrolytics used and their less than transparent presentation helps disguise it. The use of analogue sources at the time will also have helped the situation. This resonance accounts for the less than perfect top end by modern standards, and which I find particularly noticeable with digital sources, all my opinion here. Polyprops in the mid range really emphasize this top end weakness even with ESR compensating resistors fitted and without them sounds just plain terrible. As a longer term project I am now looking at what can be done to reduce this resonance effect. I have fitted Seas Tweeters (reversible), shown above, not that there is anything wrong with the HF2000, its just that the ferofluid damping in these modern units seems to reduce the resonance slightly and play cleaner, you have to adjust cap values to suit. I will be moving the circuit to the lower chamber and away from the big Bass driver magnet and re-positioning the coils to reduce interaction. The early air core coils that are fitted to my speakers are loosely wound and could be a source of mechanical resonance so new tighter wound and varnished coils will be used, again for originality the Celestion x-overs will be removed and kept intact, should I ever want to sell the speakers. I will try and isolate the resonance, which seems to be generated in the mid range band pass circuit and plays at a frequency in the range of the Tweeter and is still present when the mid driver is disconnected. So I will be splitting the circuit into its three separate filter components and isolating them. Each element will be wired separately back to the Amp (Tri-Wired), this may or may not help but is worth a try. To this end I am working on a design for a Logo/label to include three sets of binding posts which is shown above. The speakers are probably sounding as good as they did when new, maybe better, but that doesn’t mean I can’t improve on this, without the financial and constructional constraints which the original engineers had to deal with. I think it’s worth the effort as I really like the overall presentation of these speakers.