Ah yes, using Hercules - all bass trombones - would to some extent explain the darker timbre.
And ok, now I understand there is phasing. Also, this might be where the trombone timbre is darkened even more, as some fequency bands are reinforced while others tend to cancel, Whatever the exact causes, the dark trombone timbre contributes to a collision/overcrowding issue in the low mid frequency range, where the low strings and low end of brass are all competing in the same spectral space.
I did hear some compression starting to work hard when trombones lead the whole orchestra into the huge thematic passage, and I have no doubt it's not easy to tame (insofar as taming is needed for audio reproduction) this leap into mighty fortissimo. Not sure what I'd do there - perhaps try a very long gentle knee on quite a low threshold at low ratio (mastering style), avoiding any hint of the slamming effect of compression kicking in abruptly.
I'm surprised you didn't use Zodiac for the trombones' tutti line. That was where I thought Zodiac could really shine; even trounce Berliner Phil's rendition. After all, trombones have the star part in this magnificent overture. But on thinking about it, given that Zodiac's 9 trombones are all tenors it might be tricky to mimic the timbral balance of the 6 trombones in Berliner Phil's recording. Though it's almost impossible to see in the video, I'm guessing Karajan simply doubled the usual 2 tenors + 1 bass trombone specified in the score. To match that tenor/bass balance of trombone timbres, perhaps adding Hercules's 4 bass trombones to Zodiac's 9 tenors might do the trick. (And then of course there'd be a problem to be solved at the end of the overture where trombones play in 3-part divisi for the closing 13 bars - but let's not go there, lol.)
Problems, problems - ah the endless joys of digital emulation ... lolol.
I'm in awe of your energy and get-it-done approach, Paolo. Although a bit burnt out on several fronts these days I haven't lost my essential passion and enjoyment in the ever-improving realm of digital emulation. Thanks ever so much for brightening the world with your projects.
Laters.