Low in pitch i guess. There are high and low frequencies. A sub bass is pretty much only low frequencies, so if a growl also has those low frequencies it can make the mix sound muddy.
Also yea I agree it is very mudddy, I need to put filters on my chords, and a bit on the Sub bass to make it more quiet, and another Quantum on the main growl. And maybe replace the other growls with something cleaner sounding.