I'm no absolute expert on those, but about Zakk I remember interviews where he was talking about "discovering" 6550 and EV after having been using his 2203 for some time, and that he was initially using EL34 and Celestion "70w" with his Marshall. Don't know the exact time frame for the switch though.
OG Judas Priest post 82 I have little ideas to be honest, again only remember Glen talking about making the switch to "JCM800" after the Screaming for Vengeance era.
Prior to that I mainly associate them with older 4 input NMV Marshall + different kinds of boosts in front.
For Slash, I'm pretty sure the Silver Jubilees were mainly for touring in the G'n'R era; Appetite indeed was the infamous Caswell/Levi modded ST/SL 4 input Marshalls, but I remember Use Your Illusion to be the Levi #34 2203. I think that's the one Slash still has and they looked at when designing the AFD signature model.
But anyway, yes I'd agree the 2204/2203 are some of the very best amps of all time; they managed to build on the older classic NMV amps and make it one of the main bases for a lot (most?) of rock amps that have come ever since.
Well, like I said, I could be wrong about all of that.

Marshall made the switch to 6550 for the U.S. market in the early '70s, somewhere around then, apparently because they were having too many EL34 failures. I think Canadian models still came with EL34s, though. IIRC, the 2203 was introduced in 1976. Marshall switched back to EL34s sometime in the mid-'80s. So, chances are that any 2203 in the U.S. market that had EL34s had been converted from 6550s or imported from Canada or Europe. I remember that there was a lot of debate at the time as to which was better. So, it's entirely possible that Zakk experimented with both. In any case, the resulting tone he achieved starting with NRFTW became a benchmark for metal that continues today and one of the best 2203 tones.
You're right, it's hard to say about Priest. I do remember them using JCM800 Super Leads with various boosts. Their tone seemed to change after SFV, and no matter what they were using on DOTF, Defenders was a pivotal point in my youth. I still think that's one of the most perfect metal albums for me, not just the tone, but all the twin guitar parts. Right up there with Piece of Mind, Powerslave, and Master of Puppets. So many great albums came out during that period of a few years between around 1984-1987.
I'm sure you're right about Slash, too. I forgot about #34. Just curious, what do you think about the rumor that S.I.R. #39 being the same amp that Lynch used on Under Lock and Key? Personally, I don't really care. Those are both great sounds, great albums, and great players, and different sounds, as to be expected. Still, though, I can kind of hear how they could have been the same amp.
I'd have to really think to come up with a modern amp that doesn't have at least some 2203 DNA in it. What Marshall did building on their original sound really was ahead of its time.