I noticed that Death started using less gain on their tone later on and it just doesn't sound as heavy to me. Human sounds really heavy to me.
It's not gain, it's the mixing IMHO. Scott Burns has its ways to make death-metal sound as it should.
Here's a quick Scott Burns mixing credit list :
Obituary - Slowly We Rot, Cause Of Death, The End Complete
Sepultura - Schizophrenia, Beneath The Remains, Arise
Napalm Death - Harmony Corruption, Suffer The Children
Atheist - Piece Of Time, Unquestionable Presence
Cannibal Corpse - Butchered At Birth, The Bleeding
Deicide - Deicide, Legion, Once Upon The Cross
Death - Leprosy, Spriritual Healin, Human, Individual Thought Patterns
Coroner - No More Color
Suffocation - Pierced From Within
Monstrosity - Millenium
Cancer - Death Shall Rise
etc. etc. Pretty amazing track record !