The way the guitars on RTL and MoP make them sound really weak and thin compared to modern standard.
Sure, the frequencies they scooped out on those early Metallica records added a fair bit of grind to the tone, but at the expense of a lot of body.
Guitar is all about mid range dude, don't scoop out those mid frequencies like they used to.
Listen to, Gojira or the latest Between The Buried and Me records, and compare them to the old Metallica stuff.
For what those guitar tones lack in the old Metallica rhythm guitar grind, they make up for by having significantly more body to the sound.
Where the old Metallica stuff was more of a low end thump, BtBaM/Gojira is more of a mid range "shoved in the gut and then some" sound.
This really makes it sound a shit ton heavier and in a live situation especially, makes it way easier to cut through.
Even Metallica themselves, once the "gods of low mid scoop" tone had to resort to adding back some of those mids to their tone when they played live, which pretty much proves the point of how mid range is far more important than low end when it comes to guitar.
You have a bassist for the low end, so you may as well keep those mids and upper mids fairly high anyway to get the best out of the bassist's playing.