Just like cooking a meal requires a precise knowledge of each ingredient taste and flavor, mixing instruments sounds requires a precise knowledge of their “taste” and “flavor”. Taste and flavor, in the sound domain, are timbre and dynamics.
A musical sound (a note), has a fundamental frequency, and some harmonics. A musical instrument thus generates frequencies in a given interval that go much beyond the fundamental frequencies, and these additional frequencies give some personality to each instrument.
The problem with a band is that some instruments generate frequencies over the same ranges. If the harmonics of the bass are in the range of the singer’s fundamentals, and if the guitar plays in the frequency range of the singer too, the singer’s voice is drowned in the usual bad live gig mess… That’s the frequencies overlapping.
But frequencies without dynamics are nothing. The way all these frequencies raise and decay is at least as important as the frequencies distribution. In other terms, if you find a way to cut some frequencies in a guitar sound, you will still recognize it’s a guitar. But if you find a way to alter the dynamics, let’s say with a very slow attack, it will sound more like a violin than a guitar.