Interesting post, thanks Xanthe.
I’ve been going through a similar dilemma having tried my old rather unique JC-1 speakers recently to see how good they sounded with my current system.
There’s a separate thread on this (https://community.naimaudio.com/t/poorly-speakers-no-hf/29455/1) but to cut a long story short both tweeters failed within a very short time of playing having not been used for 15 years or so.
With the 851100 being unavailable now in the UK I spoke to a speaker specialist and they advised that the 852100 was an ideal replacement and very little surgery was required. It was also encouraging to learn that this is a new model which should be available for the foreseeable future. The old model was around for 35 years.
I’m pleased to learn that your cabinet modification went well. The specialist mooted the same that it would be quite straightforward with just a notch being required due to the terminals being down the side. My speakers don’t use threaded inserts so that should make things easier if I go down this path.
I had decided to park the project as I don’t really have a use for them at the moment but a pair of used D2008/851100’s have come to light so I have obtained them and will refurbish as is for the time being. I’m going to replace all the capacitors in the crossover as these seem to be a likely cause of the failure due to them being 35 years old! The tweeters have arrived and I await the capacitors which have been ordered.
Did you consider the Hiquphon OW2-92 tweeter? This is being sold as a significant upgrade and looks like a drop in replacement. They are quite pricey though so I may hold that thought for if and when I bring my speakers back to mainstream use. I do know that a Hiquphon tweeter was used in my design of the speaker, a friend has Hiqupons in his; I was a friend of the designer and had prototype pairs hence the variations.