They can be modded out for people so inclined, the power sections are similar, but it's essentially like having a tube screamer built into the amp. The Silver Jubilees also have clipping diodes as well, and that's one of the best regarded Marshall amps of all time. I have 2 of those, that's what you hear on my recordings typically if its a Marshall.
Also of note is there are two different 900 series. The reverbs ( like the 4100) are the ones with diodes, the 2100 SL-X is literally a 2203 JCM 800 with a third preamp tube for more gain. In the context of the times, it was all about hotrodded Marshalls, so that was the thinking. Interestingly enough the 4100 is pretty universally recognized as having maybe the best cleans of any Marshall.
The other factor is the speakers and cab are 60% of the sound. If it's not a 4 x 12, none of them are going to really be "that" sound. Combos just won't have the thump.
Thanks for coming to my late 80s early 90s Marshall Ted Talk.