Natural: lack of telltale electronic signature that signals a “synthetic “ quality to the perception. Doesn’t have to be “neutral” as this is often a label given to systems that are full of those telltale sonic signatures, such as a grayish tonality and flat dimensionality.
One caveat: we all listen differently. Thus, even two experienced listeners to unamplified live music may not even agree if something sounds natural or not. But that experience is a good start that should narrow the field of disagreements.