We are now at the last stop of our journey. The Mastering stage. Contrary to popular believe this is one of the less significant parts of the whole process concerning the overall sound of the song.
If you have to alter the sound in a fundamental way in the mastering stage, you should really go back to the recording or mixing phase.
The main goal of mastering is to bring the song, the ep or the album together and form a coherent work of art. Mostly making careful changes to the frequency spectrum to get an even listening experience across all different devices. So the song works in a car, a hifi stereo set, earbuds or a bluetooth speaker. At the same time the dynamic range gets reduced with clipping, limiting and compression so there is an even level of perceived loudness and no major loudness changes that don't serve the evolution of the release. Fact is, when we hear the same song just a little louder we instinctively think it also sounds better. So there is real utility in making your song at least comparatively as loud as other major releases, so it doesn't feel less powerful when put on a playlist with other songs. Nevertheless you also can easily go to far and risk digital distortion and artifacts, when it is converted for the different distribution plattforms.