The Mark III predated the release of the SLO by a couple years. Metallica was already using the IIC+ by then. Master of Puppets recorded and released well before the SLO was built.Soldano took a Marshall style preamp and added an extra stage, a cold clipping stage (39k) and a lot of filtering (1uf cathode caps) and the big one IMO, the filtering of the signal between the first and second stage.. 1N/500k.. (by memory) that reduces a lot of bass early on and allows it to stay tight, despite being so "gainy". And also two cathode followers, with one for the loop (which isn't essential to high gain tone IMO but yeah). With the two channels, you could go anywhere from clean, to Plexi ish gain to modern high gain.
Then the Mesa with the Dual Rec or the later versions of the Mark series.
Very good write up, I still have a soft spot for mesa and would like to point out that while Soldano was first, the tone was in the Marshall style "EL34" camp, Mesa's Modern Marks + Rectifiers were firmly in the American/6L6/Fender style, their own unique twist not a copy as said above.So take your pick I guess. IMO, Soldano was the most influential in shaping the "high gain" tones of the future. Pioneering them.. and being a foundation for later people to copy etc. You can say that Peavey 5150 or Dual Rec copied this amp.. but indirectly.. most high gain amps copied the "principles" used in the tone shaping, but giving them their own twist.