Holy crap, it's gonna take the OP a hundred years to UNLEARN everything he's been told in this thread!
I've owned and/or played almost every model Marshall ever built.
#1 None of the amps mentioned in this thread ever came with 6L6 tubes. The USA versions shipped with 6550s and the Brit versions shipped with EL34s. They're easy to change if one wants to. Personally I like the 6550s better, they hold together better at high volumes and have bigger lows.
#2 Two channel JCM800s such as the 2210 have LESS balls than single channel models like the 2204 and 2203. I have found the single channel versions to be much better all-around. I think someone is confusing these single channel models with non-master vol models like the 1987 and 1959 although they can scream pretty good too if pushed with the right pedal or run through an attenuator. The 2204 and 2203 do not have to be modded to get the sounds the OP wants. The right volume settings, the right cab and the right OD will take your old Marshall to Screamsville. Here are my two favorite pedals to kick an old Marshall in the ass with:
Mesa V1 Bottle Rocket:
Marshall Drivemaster:
#3 The JCM 800 is not necessarily
better than the 900, just different. The 900 SL-X is a stand-out among the 900s, it has more available preamp gain than any of the other amps mentioned in the thread. If I was looking for a purpose-built high gain Marshall this would be the one I would try first. The JCM800 is not exactly the first amp I would plug into for LOG covers.
Last of all, I have always preferred 100 watters to 50 watters. I like the fact that they stay tighter when they are cranked.