This is what Trace at Voodoo said:
2203/2204 - As a whole the JCM800 line is not all that consistent. There are numerous variances. In direct A/B comparison; Some are noticeably brighter than others, some are more distorted, some have more crunch, other are grainy, etc. For example, the early 2204's ran on lower plate voltage and have more gain than most others. The trade off in this case is in the form is less headroom.
It is true that the very first 2203's were much the same as the JMP Master Volumes however there were not very many of these made, making them hard to find (look for 6 filter caps on top of the chassis verses 3). Generally speaking power supplies varied and filtering varied, IE: later models have V2 and V3 sharing a filter cap with the 47pF on the bias splitter side verses the plate side, etc and so on. The concept with 100 watters used to be (and some still do) put filter caps in series to increase the voltage rating. Generally speaking most do not consider this to be a necessity on 50 watters. Often times the filter caps on the 3 can-cap 2203's are bulging on the top but the 6 can-caps do not suffer from this.
2205/2210's - The dual channel models also have their share of differences. Models made in 1983 have a lot less gain than the later models and usually about the same or less gain than a majority of 2204/2203. There are models in 1984 that also have less gain and a majority of these have red silk screening on the rear panel (models to be imported to Holland, Canada, etc). On some models the clean channel bleeds through on the gain channel. Circa 1985 these models were redesigned to have more gain. The later models had a bit more gain and this occurred circa ‘87/88. On average the models after 1985 do have more gain than a 2204/2203. The models with black silk screening on the back are the same as the later models that followed but the models with red silk screening were identical to the models made in 1983. It should be noted that the dual channel JCM800's gain channel is not the same as the 2204/2203. It is very different in design and as such one does not sound like the other.
Many people crank these amps and use an attenuator, under these conditions the filter caps can easily see voltage spikes past their voltage rating. This is one of the reasons why the older JMP Master Volumes are preferred. IE: the filter caps are rated for 1000VDC verses 500VDC. The bottom end is deeper/richer and the over all dynamics and touch response are much better. The 3 can-cap versions can be modified for the same filtering and we have done it for several years.
This is what Jeff Seal said:
Marshall made the most radical design change on the channel switchers around '86.....(no other amp has undergone such a complete redesign in Marshall history) The early version's have a pretty good clean channel, but the OD is more along the lines of a "*umble", the later versions have IMHO the most killer OD, but the clean(normal) channel isn't particularly clean, but usable.
The 2203/4 had some simple changes made to them, they are essentially the same design. Of course, you have to play through as many as possible to make up your own mind.
Michael Scenker - Early 2205
Tom Morello, Dino Cazares.. (FF) - Late 2205
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I would like to add that I believe it was around February to March of 1977 that Marshall began adding an extra cascaded gain stage to the JMP (a simple one wire mod). You can see AC/DC playing these in the Dog Eat Dog live performance from April 3, 1977 and supposedly they received some of the first ones that Marshall made. This was done because people wanted more gain after the addition of the master volume 2203 & 2204 that happened in lat 1975 or early 76. Again around mid-1979 Marshall made another change to the circuit which resulted in more gain. This extra gain had more to do with what people usually describe as the difference between early JMPs and early JCMs. In late 1980 or early 1981 when Marshall's 15 year distribution contract with Rose Morris expired they decided to cosmetically update the whole line to the JCM look.
I have read books on this subject, and somewhere there is a photo of the Marshall parking lot and Jim Marshall had a jaguar with the license plate JCM 800. JMP stands for Jim Marshall Products, JVM stands for James Victoria Marshall, and JTM stands for Jim and Terry Marshall. As for JCM, I've heard it stands for James Charles Marshall or Jim and Clarence Marshall, take your pick. Here is some further reading if you're so inclined:
http://www.solodallas.com/the-vintage-marshall-guide/http://www.vintageguitar.com/1974/marshall-amplifiers/