Agalloch definitely have good production, though unfortunately low dynamic range. Swallow the Sun varies from album to album. I've use DT880s, FA-002ws/003s. FA-011s, HD600s, Fidelio L1s, and HD800s so I wouldn't call what I've used low-fi, but more mid-fi. HD800s were terrible for metal and I assumed that it was similar for most very high fidelity equipment, but I this thread seems to show that the HD800s are specifically bad at metal.
I keep looking at the mid-fi champions: K712, DT880, HD600/650, Mad Dogs, and Fidelio X2s. However I'm terrified of spending that money and wanting to upgrade again.
For Candlemass, Black Sabbath, Ahab, Swallow the Sun I would go for Mad Dogs. Gojira, Septic Flesh, Ahab (again!)), Swallow the Sun, and Agalloch - K712.
I digress, guys what are your favorite albums/bands?
My personal "The best" list for year 2014
Agalloch - The Serpent & The Sphere
Anaal Nathrakh - Desideratum
Animals as Leaders - The Joy of Motion
At The Gates - At War With Reality
Autopsy - Tourniquets, Hacksaws And Graves
Behemoth - The Satanist
Bloodbath - Grand Morbid Funeral
Blut Aus Nord - Memoria Vetusta – Saturnian Poetry
Conan - Blood Eagle
Cynic - Kindly Bent To Free Us
Earth - Primitive and Deadly
Eyehategod - Eyehategod
Falconer - Black Moon Rising
Fallujah - The Flesh Prevails
Hail Spirit Noir - Oi Magoi
Khold - Til Endes
Mantar - Death by Burning
Mastodon - Once More 'Round The Sun
Misery Index - The Killing Gods
Monolord - Empress Rising
Nadja - Queller
Opeth - Pale Communion
Origin - Omnipresent
Pallbearer - Foundations of Burden
Quintessence Mystica - Duality
Sabaton - Heroes
Schammasch - Contradiction
Septic Flesh - Titan
Serdce - Timelessness
Slugdge - Gastronomicon
Soreption - Engineering the Void
Sunn O))) & Ulver - Terrestrials
The Contortionist - Language
Thou - Heathen
Unearth - Watchers of Rule
Vader - Tibi et Igni