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Mastering
Assuming you aren't working with a bad mix, mastering is fairly objective.
Recommended mastering studios
https://www.cpsmastering.com
http://breakpointmastering.com/
https://michaelwynneaudio.com/mastering/
Reddit user u/frankiesmusic
Mastering WIKI
https://www.reddit.com/r/mixingmastering/s/nmhGf5W0zj
https://www.reddit.com/r/mixingmastering/s/nqtGH4SiQW
Recommended mastering studios