Apart for how bad I may have used it, I'm not sure I dislike the Großer Saal pack. I think it is true to the few recordings I can find clearly referring to it. More than "boxy", I would say that this type of hall is "warm". That is, it has not that sparkle that is in more modern halls, like Teldex or Synchron.
As for any boxyness in my mockup, I could refer it to the use of compression. Yes, I admit I use compression! Without it, I feel the result is too uneven. I use parallel compression, to be sure the sound remains as natural as possible – but compressed sound will remain dominant.
In the new version I uploaded, I replaced the compressor with a different one, with the most transparent settings I could achieve. And parallel compression, here too. I don't know if it sounds more open. It's different, probably smoother and more pleasing, but I don't know if also more natural.
I also shortened the MIR reverb a bit (from 2.2 to 1.8 ms). All considered, I prefer, as you, shorter reverbs for this type of music.
Paolo