You've got to try moving away from boxes that produce so much noise in the high end, you want booming mid range, especially low mid, the high end doesn't need distorting, boost the low mids and you'll get the heaviest sound...
But that's just me perhaps, I have come to hate the sound of white noise in a guitar sound...
Listen to Neurosis, Pelican, Capricorns, bands like that, so much heavier than these bands where you just here cheeeeecheeechaaa etc...
Brutality comes from punching someone in the balls, not by making buzzing noises in their ears...