The muted, downtuned, aggressive riffs of BRUTAL DEATH METAL are as heavy as you can get; heavier than any bluesy stoner riff penned by a bunch of high Black Sabbath wannabes. heh
I don't see how muted guitars are heavy because they're muted. I'm not against palm mutting it's just the more muting the less heavy something sounds, you're not holding out chords/notes for sustaine to see how heavy guitars are.