Don’t worry. Be happy. Blast both of them (well, maybe the string quartet could be set to a natural sound level). But to me, full orchestral and opera (preferably Blumlein-miked) are the ultimate tests.
There really is no instrument that is any “better” than another for assessing truthfulness of timbre (within a given frequency range). It depends on just how intimately familiar one is with the sound of a particular instrument; and personal preference for that particular sound. Every instrument or combination of instruments has its own unique timbral characteristics which pose unique challenges to the process of record/reproduce.
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