Mix is too bright
While mixing some people tend to overbrighten overall sound. Boosting high frequencies for sure can make an mix more attractive, but you should not carry too far. Humans ear way too susceptible to sounds at 2 - 5kHz range, and there is a lure to increase this range to add transparency to music. Exagerration might turn track to a super harsh and metallic one. How to stay away of it: Make compromises by EQing. If required a certain element to be more vivid in a track, when working on a song engineer has to clean up more space while processing other sounds. EQ automation can do a job if needed some sound to be better audible at specified part of a song. Inserted compressor can help to deal with harsh percussive sounds. Transient designers VSTs will also help to decrease hardness of sound. Experienced engineer surely will advice to you that it is way better to take away it than to add.
Tune is too muddy or too plain
The surfeit of low frequencies can threaten with issues at the stage of both professional mixing and mastering services. How to prevent it: You should process instruments which need low frequencies (bass drum, bass) first. Then you should set a low cut filter on rest of tracks to get reed of unnecessary low frequencies. As result you will make space for the tracks which really need low frequencies. When mentioned sounds are particularized, clean up separate room for them in overall mix. Be very careful while filtering bottom ends as their deficiency on vocal or on the main instrument can deprive it of life. Moreover, by sub lows filtering (30 Hz and lower) you can bring out extra space for bass. Experiments are the solution to such kind of issues. In the course of time you can develop your own techniques to fight against them.
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