Hi Charger.
I used several models of Marshall Heads and Cabs from 1974 to 1976, again in 1988 to 1994.
In the 70s, my main heads were two Marshall 1959s,100 watt heads primarly, with six 4x12 cabs. For a time I had another model as a back up but never had to use it. It wasn't a 59 but I don't remember what it was.
The heads were tube and if I remember right were made in the late 60s. Not many extras. Pedals were used.
In the 80s and 90s I used one of the same model heads but it was made in 1972, and two JCM 900 1960A 4x12 slants.
Other times I used Peavy, Ampeg, Fender, smaller Marshall amps and so on.
So,,,
to answer your question, "Do I think Marshall makes good equipment?"
"Fuckin' 'A' I do".
And, "No!!!, I didn't 'answer just like that'".
It took some years of using the equipment, using other brands to form my opinion on what amp & cab I would use outside again and recommend to others.
By the way, I used a Gibson SG with all of them. I liked the sound and stability of highs and lows from the combination.
You shouldn't waste your time trying to figure out something as complicated as why Marshall dominated for thrity years. I recommend you try some older Marshall equipment, or some newer equipment. Judge for yourself.
Like I said, I used Marshall equipment outside most of the time. If I were to go on tour again, I'd use the same brand. It was reliable, stood up to months of heavy use and being moved and set up all over North America. I don't recall any need for repair other than a tube replacement, if even that.
The sound was what I wanted, fat and clean. I play mostly blues and rock so those combinations of heads and cabs fit my taste.
The sounds carried in places like the Daytona 500 raceway, The Tangerine bowl, Rogers Collisum, in canyons, open fields and wooded areas, and other outdoor gigs.
Inside, I use Fender and Music Man Amps with a variety of cabs. Depending on the size of the building.
When you get that domination thingie figured out, lemme know. I'd like to read about it.