This isn’t exactly true for this particular example. The 2203kk, and other 2203/4’s pre 1986 sound different because they are somewhat different. Marshall revamped the design a bit for his signature model, and all the re-issues are more focused on recreating models pre 1986. Back then Marshall was using components with the same type and tolerances as they largely do now. You only really see a lot of that inconsistency with Marshall in the 60’s and early 70’s, and I’ve seen my fair share of Marshall’s on my bench. The biggest reason kerry’s Amp sounds different is because around 1986, when they introduced the horizontal input 2203/4 models, Marshall changed things a little in the production. They adjusted some things in the power supply mostly to save costs probably, but secondary to make production more consistent between both 2203 and 2204 models. This adjustment to the power supply increased the supply voltage to preamp section by about 100V which resulted in a somewhat even brighter and tougher cleaner sound, with less compression at higher gain settings than the pre1986 vertical models.. The old vertical models had a pre voltage of roughly 280vdc which does brown things out a bit. whereas these models had one of over 380vdc. This continued until it was discontinued in 89 or 90. All the horizontal input amps from this period have the higher preamp voltages, and sound brighter and cleaner like kerry’s Original 2203.
I owned a red ‘86 at one point when I was much younger, and could never figure out why it didn’t sound the way other 2203’s did. It didn’t have the juiciness I expected, and was cold and stiff instead. I figured the lore was true. It wasn’t until I first started out in electronics that I took look at what was really going on, instead of just matching parts for parts. And that’s when I found that difference. Dropped the voltage to the preamp to pre ‘86 spec’s And there it was! The amp was stolen from me later. Don’t blame them, still bitter though.