Daniel C. answered 12/01/20
Music Producer shares secrets. Pro Tools and Mac User Expert.
First and foremost....A great room that has a flat response and goes down to very low frequencies.
Mine goes smoothly down to 17 cycles. I measure it regularly. I show my frequency response to clients. Lots of studios won't do that or even know how to measure it. Ears and perspective are the other best tools. If you are becoming a mastering engineer take a critical listening course like the one I teach at Berklee. If you are further along then read:
https://www.thriftbooks.com/w/mastering-audio-the-art-and-the-science_bob-katz/311147/item/5829119/?mkwid=%7cdc&pcrid=448938662877&pkw=&pmt=&slid=&plc=&pgrid=109793399012&ptaid=pla-894509663962&gclid=Cj0KCQiAzZL-BRDnARIsAPCJs73YHQWtvv3iXq-Vfrs_p7Dri0KdA4yA-Y4lPROpwWpEsXq2OWuKmn0aAkE2EALw_wcB#idiq=5829119&edition=5782131
If you want mastering lessons or audio gear consulting, I can do that through my Music Production portal. I love finding discounted equipment for my students.
All my best to you.
Daniel