First get the midrange right
As demonstrated by the Fletcher-Munson curve, the mids change very little at different volumes, leading many to believe that the mids are key to a balanced mix. From here, you can treat lows and highs as extensions of the midrange.1
Mix on NS10s or a Mix Cube, which emphasize the mids.2
Alex Tumay, Jaycen Joshua, and Michael Brauer agree that 300Hz is “where the song is.”3
Gregory Scott says 800Hz is the foundation of the midrange. This can give a mix some “sea legs.”4
Jack Joseph Puig says the soul is in the midrange. Start your mix just focusing between 200-4kHz.5 Pay special attention to 1k-5kHz. Boost instruments not just at 4k-5kHz, but also 1k-2kHz.6
CLA says that the subs take care of themselves if he gets everything else right. Low-end power comes from the low mids.6 This may be an example of Don’t boost what you can’t hear.