Most mixes suffer of what we call “Mud”(300-400Hz)
Especially on guitars the frequencies between 200Hz to 400Hz, sometimes even up to 500Hz are killer for the guitar sound. But this is not only valid for guitars, this happens with every instrument. And when you sit everything in the mix it feels dull, thick like you put a blanket over your speakers, you have to cut those frequencies out. That’s why in the channel strip I like to put two EQs. One, is a corrective EQ and the second one is to bring balance to the sound.