My frustrations with HF of this amp reminds me the Agatha Christie novels:.
however, like anything else it has an end, even a temporarily end:.
After some experiments and still being dissatisfied with the HF I ended up consulting with Dima. He spread me a theory according to which the HF was killed by my coupling Silver Mica cap. All my arguments that silver mica work OK he demised claming that it is known how mica works in parallel application but generally people do not use it is serial application. Well, it sounded like a reasonable argument. I personally did not believe him as I firmly stay that sound (instead of The Sounds) does not come from sonic signatures of the parts but from the global topological implementations.
However I decided to try. I placed between the stages my default 2uF Cube 950 (with speaker lever crossover) and it was like return back to home after a long journey. The amp immediately paid back with beautiful and VERY extended HF. In fact they are even more interesting and more extended then with a regular Melquiades! Do not forget that now the S2 driver does not see the upperbass's coil and it makes it sound VERY clean! It was exactly one of the main objectives that I visualized for this project.

Now, I still do not know if he Dima was right or wrong about the Mica. It is possible that your proposal of high impedance does play a roll in there as well. Or perhaps Dima was correct and the only cap should be blamed for anything. Dima made me, contrary to my weak protests, to bult my own Cupper Foil with Teflon cap that I most likely will try soon in the HF channel.
Perhaps air or vacuum will be next. HOWEVER it still will NOT help in there if the high impedance was the reasons of HF killing.

Frankly speaking I so love the Cube 950 that if I get the correct value I would not even experiment with anything else. The lower value I can get might be 1000pF (.001uf) that would require 40K bias resistors. I do not think I might go so low:

I really begin to hate all of thing! When will that all soldering stopped!!!?