The pay off was that the dedicated crowd at the Trusts Arena got to see something special as the bands, out of their comfort zone, invoked the spirit of rock and roll and dug deep.
With a minimal light show and no visual backdrop, it was all about the music. Both bands killed.
French metal legends Gojira were hard edged, industrial at times, but with prog detours and noisy art rock breakdowns worthy of Sonic Youth at their most manic.
Lead singer Joe Duplantier is charismatic, with the chiselled look of Chris Cornell about him. He explained the gear scenario and won crowd early.
Duplantier even dropped a little French existential philosophy into his banter, reminding us there is only the "here and now", as if the bludgeoning sonic assault wasn't enough to keep us focused
Gojira are a tough act to follow.
But Mastodon have earned their spot as one of greatest hard rock and metal acts in the world right now.
Hard rock or metal? To borrow a line from The Blues Brothers - they play both kinds of music.
With incredible skill - and all four members sharing vocals - they range across metal sub-genres from the darkest of death metal roars to sludgy grunge crooning, classic 70s era wailing and even some surprisingly sweet harmonies.
The band took a few songs to warm up, possibly getting the feel of the unfamiliar gear, but when they hit their straps you could see them smiling and you knew it was going to get good.
Drummer Brann Dailor drove the band to something special. Without ever indulging he took the lead on several occasions. At times all you could do was lock into the drum track and let the complex rhythms consume you.
Dailor is as crisp nimble and tight as a jazz drummer but still manages the thundering heaviness that genre requires.
Up front the rest of the band let rip. The two pronged lead guitar attack soars like Lynyrd Skynyrd in full flight.
Front man Troy Sanders, on bass, hams up the metal poses - with his now greying mane of hair and Gandalf-length beard giving him a wizard like aura.
Mastodon in top form are worthy of comparison to Led Zeppelin, Black Sabbath or Metallica.
They take you to a primeval place. They don't do ambiguity.
They roar like paleolithic hunters, screaming into a storm of thunder and lightning, cursing the gods for the suffering they have bestowed on the world.
Then they shift a gear. Suddenly you're bouncing to a dirty rock groove worthy of Guns and Roses or Queens of the Stone Age.
Live, the poppier, more radio friendly numbers off new album Emperor of Sand, shake off any softness that might have crept into the studio mix. Show Yourself just rocked and Steambreather provided an epic close.
But for the fans, classics like Blood and Thunder and Black Tongue delivered a megalithic dose of metal magic.
This is a band still evolving, still at its peak. Still brutal.
