There's no easy answer to the question of good 800's and bad 800's, except that it's strictly on a case-by-case basis. That's the nature of Marshall's manufacturing inconsistency over the years.
People can bitch and moan about this all they want, but it was primarily Marshall that wrote the book on Rock n Roll amplification, and they didn't exactly have a shitload of reference points to go by, so in effect I'd say Marshall made "mistakes" or "less than best" decisions in manufacturing and design through the years, and that companies like Mesa, Rivera, Peavey, Randall, etc, learned from these in order to better their own designs, so if you ask me, I think many major amplifier manufacturers owe a huge debt to Marshall for showing them what they could do and/or do better, as well as perhaps what NOT to do.
The biggest deciding factor in tonal differences between Vertical and Horizontal input 800's was the power supply configuration. Vertical input 2203's had 6 cap cans, or 12 individual capacitors. As part of cost cutting, and also the implementation of a standard spec power transformer for both 100w and 50w models (previously there was a PT with different secondary voltage specs for each model) the filtering arrangement changed and amount of filtering to the plates and screens increased, and also the plate voltage in the preamp increased by as much as 100v. This caused the Horizontal input 800's to sound "cleaner" and perhaps more polished but less full than the vertical input models, which had a more raw sound because of the filtering arrangement.
