Jecklin tweak
Great review! I own a pair since 1990 and have been happy with them ever since for all the reasons you mention: wearing comfort, spacious soundstage, euphony, generous bass (though admittedly not very deep below 80 Hz) and possible loudness (very dynamic peak reproduction).
Having seen your photo's I opened them up for the first time and noticed the large open spaces inside, between the drivers and the side 'walls' of the plastic enclosures. Lots of resonating cavities and flexing panels there!
So I filled those up with plasticine clay (from a toy shop). I dared do this, having damped the underside of my Linn turntable platter with this stuff a few years ago with very satisfying results: it opened up and tightened the sound of the 'table a great deal. That plasticine has kept its damping properties up to now and it hasn't dried out, nor did it adversely affect the materials of the turntable, as far as I can see.
The Jecklin is reborn! All vagueness that you mention and I recognized in your review is gone! Localization and tightness are really excellent now. Fast peaks are much less fuzzy, cleaner and more dynamic/loud.
So it seems the limitations of the original Jecklin are primarily in its mechanical construction, and not so much in the drivers themselves, nor in the transformer box.
The phones have gained some weight by the plasticine, but they remain very comfortable. I really hope they stay healthy for a couple more years. If not, I'll have to buy something else, that is at least as good. What do you recommend, the Precide or the Omega 2?