optimal level totally depends on the situation... including other people in the room, your purpose, time of day, are you engaged in other activities, etc. ... and depends on the speakers, etc., too.
some speakers need to be "turned up" to come alive... I'd never want such speakers. Others do very well at even quiet volumes; I'm constantly impressed by the dynamism and richness coming from the Klipsch Heresy at very low volumes, for example, and some of the most interesting listening experiences I've had with the Epi 100 over the years have been at low volumes late at night: they have clarity and sparkle.