Here’s a quick tip that will help you clean up your mixes and make them less muddy. Add a high-pass filter to every track. Yes, even bass and kick drum tracks.
The reason for this is that unwanted low-frequency information will quickly muddy up your mix. Even tracks like triangle and glockenspiel will likely have low-frequency noise that may be inaudible to you. If you’ve got 20 or 30 tracks in your mix, all this mud will build up and overwhelm your low end.
Because of the way our ears work, the low end data in your mixes has to be significantly louder that the highs and mids to remain balanced. Thus, most of the energy in a track will be concentrated in the low end. This means that compressors and limiters will react to this low-end data first, and if a large part of it is non-musical rumble and mud, this will adversely affect the way the compressor works. Cleaning up the low end will mean your compression will be more musical and thus will sound better.