After a cold glass of water to the face from metalcore masters Converge, the tempos slow way down on this walloping set of doom metal and heavy rock. Pallbearer is an uncommonly melodic metal group, a brontosaurus lumbering from riff to riff with primordial grace as singer Brett Campbell channels his inner Ozzy. Machine Head makes the most of “Halo’”s nine minutes by constructing an intricate 12-part epic, while Sleep turns the hazy, consciousness-altering subtext of doom into text with “Marijuanaut’s Theme.”