MUSIC: Mastodon, Opeth and Ghost throttle House of Blues

“Play something heavy!” a dude in the crowd bellowed.

Mikael Akerfeldt remained unmoved.

“This is called a capo,” he said in between songs at the House of Blues at Mandalay Bay on Wednesday as he tuned his instrument. “It is probably the most un-metal thing in the world because it actually tunes up a guitar. Sorry.”

And with that, Akerfeldt led his band, Swedish progressive metal changelings Opeth, into “Windowpane,” a long, meditative number with emotive soloing and almost wistful vocals.

Opeth began in the early ’90s as a death metal band, but over the years, they’ve gradually moved away from their early influences, with Akerfeldt completely abandoning the music’s trademark guttural vocals on the Opeth latest disc, “Heritage,” which owes more to ’70s prog rock than it does Morbid Angel.

There was even some flute work, albeit sampled, on “I Feel the Dark,” a shadowy, solemn number that begins with an acoustic guitar interlude and gradually works itself into a dark, jazzy vamp.

Metal isn’t supposed to be this pretty, hence the catcalls from some crowd members who wanted the band to play some of their earlier material.

They’d get there eventually with a pair of songs at set’s end, “Demon of the Fall” and “The Grand Conjuration,” where Akerfeldt roared like a hobgoblin with digestive issues over steamroller riffs.

This same kind of evolution has marked the career of Atlanta metallers Mastodon, who followed. Like Opeth, their early recordings were defined by brute force combined with technical acrobatics resulting in a dense, indissoluble sound.

On their latest album “The Hunter,” which the band played almost in its entirety, they leaved their turgid crunch with heightened melodic flourishes and more streamlined arrangements.

Bassist Troy Sanders, guitarist Brent Hinds and drummer Brann Dailor all take turns singing, with Hinds employing a gruff bark and the other two handling the more sonorous, clean singing.

With Sanders swinging his instrument like a scythe, the group barreled forth with flying hair and clenched teeth, alternating seismic jams like “Hand of Stone” and “Crystal Skull,” which came on like mortar fire, with more anthemic tunes like “Curl of the Burl” and “All the Heavy Lifting.”

Like Opeth, they played old favorites at the end, storming through “Blood and Thunder” and “Aqua Dementia,” revisiting the past as a way to show how far they’ve come in the present.

Opening the show were Swedish occult rockers Ghost, who like their countrymen in Opeth have a sound rooted in the ’70s, though they owe more to Blue Oyster Cult than Jethro Tull.

With their frontman dressed like a demonic clergyman, complete with flowing gown, headdress and face paint, and the rest of the band in hooded robes that obscured their features, the band crafted rousing, warmblooded odes to Satan and human sacrifice.

Somewhere, Beelzebub was snapping his fingers.