The rhythm guitar wall is the hardest single problem in metal mixing. Two distorted guitars, hard-panned, already eat 200Hz-3kHz. Add doubles and you've got four. Each one has substantial midrange information. Without aggressive subtractive EQ on each layer, they pile up into a wash that obscures the kick, snare, and vocal. The high-pass at 100-150Hz on every guitar is non-negotiable. The 250-500Hz cut to remove "boxiness" is non-negotiable. The 800-1.2kHz scoop or boost (depending on subgenre — thrash scoops, prog/djent boosts) is where genre-specific taste enters.
Triggered drums in metal exist because the natural drum sound can't survive the guitar wall. The kick of a real drum kit has a 60-100Hz fundamental that masks the bass guitar's playing range; the natural snare has a 1-4kHz buildup that fights the rhythm guitars' presence. Triggered samples or replaced layers let the engineer dial in only the frequencies that survive the mix — the kick beater click and the snare crack — while leaving the lower-mid territory to the bass and guitars.
Bass in metal usually splits into two parallel layers: a clean DI track for the deep weight (40-80Hz fundamental) and a distorted layer (DI through a SansAmp or similar) for the 200-800Hz definition. The distorted layer cuts through the guitar wall by occupying frequencies the guitars deliberately leave open via their high-pass and the rhythm scoop. Without the split, the bass either disappears entirely or muddies the bottom.