Thanks, Dietz, for the thorough reply. Very appreciated as always!
I don't want to use delay, it was just for comparison reasons. I think I indeed confused myself, because there are so many variables to deal with, especially the individual capsules you can edit. For example: I tested various setups with dry volume offsets on the center capsule (in my case a Blumlein with center).
I started to balance my template from the strings (dimension strings), and this was indeed the mistake. I already mentioned in another thread that the natural volume overview isn't right on certain instruments. For example, the dimension strings had -15, now -12, and I think they need -21. Maybe the numbers have been mixed up.
Of course, you are always saying that natural volume should not be used with other libraries, but that's not true. I tried with all other libraries I have (Synchron, Hollywood, Spitfire) and the volume balancing is really spot on, except some minor things which are not right in the overview (overview vs. the implementation in MIR Pro). The important thing is you always have to use a combination of main + spot when referencing.