Mastering is about the presentation of a mix.
The importance of full range, low distortion speakers and D to A converters for monitoring is critical. Mastering is mostly done by listening for and then fixing distractions. This can be tonality, dynamics, noises or distortion. My view is that mastering should not call attention to its self.

Mastering an audio project involves fixing all the minor inconstancies. We take to the controls for the final stage in the process.