there is no baseline or reference ofr a perfectly neutral stereo system. it does not exist.
thus if one is using outlet brand a, the stereo system will not be neutral. if one is using outlet brand b, the stereo system will still not be neutral, so there is no logical way of knowing that a component is or is not (absolutely) neutral.
i think one would seek an outlet that works well with one's stereo system and not be concerned about neutrality, which does not exist and will never exist.
if you consider what a stereo system is and what you hear coming from your speakers, listening to recordings whose "sound" is unknown, chasing neutrality is a waste of time.