The 80s metal sound actually does not come into existence, formally, until 1981 when Marshall released the JCM 800 amplifier which incorporated a master volume control and unleashed the era of preamp saturation. Of course, it took a while for the 800 to become ubiquitous so, really, it is the recordings of the mid-80s that represent the apex of the 80s sound.
Metal prior to the JCM 800 (still subsumed under the classic rock rubric) are exemplified by such albums as Back in Black by AC/DC or Screaming for Vengeance by Judas Priest. In adulthood one may look back and dismiss these records as juvenile (and they are, especially the vile misogyny of AC/DC) but the guitar sounds are timeless, whereas metal albums from the mid 80s that relied on the 800 sound ridiculously dated and cartoonish; when we hear "80s metal" we think of hair metal bands.
And bands with great guitar sounds in the late 70s and early 80s, like Judas Priest, succumbed to the cheap thrills of preamp saturation -- and suffered because of it. Contrast the full-bodied sounds of Screaming for Vengeance (1982) representing the 'Plexi' sound to Turbo (1986) just a few years later where you hear the JCM 800. The sound is thin, bright, and crispy. Notice the heavy reliance on synthesizers to fill out the thin sound of the guitars in the mix. Screaming will live forever whereas Turbo will live in infamy. And once bands drank the preamp saturation Kool Aid they tended to go even further sideways into Rocktron preamps and whatnot.
The horrible reign of the JCM 800 was not displaced until Metallica made the shift from the JCM 800 on the first few albums (Kill 'Em All is a good representation of their early sound) to Mesa Boogie (best represented on the so-called 'Black' album in 1991). As Hetfield stated:
“The last time I used a distortion [overdrive] pedal was on Ride the Lightning, and it was hell. It was an Ibanez Tube Screamer like Kirk uses. It really helps his solos cut through, but it puts a shitty coating on smooth rhythm tones, and it was hard to make it not sound like a pedal. You can recognize Marshall distortion in an instant; that's why I shied away from that and went with MESA/Boogies.”
And who was to blame for this 80s sound where everybody wanted a fizzy, bright, thin sound? Was it just Marshall forcing new technology on guitarists or was there a prestigious sound on the charts that everybody wanted to achieve?
Listen to Ozzy Osbourne's Blizzard of Oz (1980) or Diary of a Madman (1981) with Randy Rhoads making two Marshall Super Leads from the 70s sound as bad as possible. These recording sound like he was prevented from cranking his amps and making up for the loss of power amp saturation with pedals. But what is the real story?
“Before recording Blizzard of Ozz with Ozzy, Randy ordered some Marshall amps direct from the factory, based on the 1959 Super Lead but customised with white Tolex and including several modifications. The most important of these mods is a change to the way the two channels interact with each other. Typically, a Marshall Super Lead would have two channels, I and II, which each have 2 inputs. Players discovered early on that they could run these two channels together by joining them up with a short cable, but the 1959RR offers an internally-facilitated variation on this idea, all in the name of increased gain. The mod, which is only on channel II, cascades both halves of the first 12AX7 preamp valve, feeding the output of the first stage into the input of the second, instead of using each half separately for channels I and II. This effectively makes channel I’s volume control function as a master volume, while turning the channel II volume into a gain control. Or if you are after a classic Super Lead sound, just plug in to channel I, throw in some industrial-strength earplugs and off you go.”
So, perhaps, we can thank Randy Rhoads, one of the most overrated guitarists in history, for the shitty sound of the 80s. I recall very well the Blizzard/Rhoads effect as a young guitarist in the US in the early 80s. Everybody wanted that thin shitty sound all of a sudden and the old Plexi amps were being sold/traded for the new 800 amps that helped to create an entire era of unlistenable music.