"Natural Volume' is [...] a suggested starting point"
Natural Volume [...] a real, objective measurement, not a matter of art or taste.
VSL has the answer. They know exactly how the VI Pro volume should be set in order for the MIR "Naural Volume" to work as designed. It's just a matter of their withholding that information from their customers.
I'm basically parroting there what I've seen someone from VSL, perhaps Dietz, state about the matter on this forum.
I don't know where 'a real, objective measurement' comes from. I guess if someone only ever uses VSL Samples and MIR Pro it comes off as a more or less naturalistic out-of-the-box experience, but I've seen this asked several times here and that's my received answer. First time I noticed it, it was 'I'm not hearing enough of this instrument' and I adjusted for it. And that isn't I think very unique to me.