for a while the 9200, like other 9000 series Marshalls, was only available with (sovtek)5881 powertubes (aka 6L6), even though they were originally designed for the EL34. Marshall switched to 5881s when they couldn't get their hands on quality EL34s. IIRC when EL34 started to become available again they switched back.
I'm guessing that for their current lineup of rack poweramps they wanted to make the differences in tubes and power very clear, hence the name EL84 20/20, EL34 50/50 or EL34 100/100.
A lot of people will identify the Marshall sound with EL34s, but Marshall have occasionally built amps specifically not with EL34s.
Some of those are currently available, such as the Kerry King model (using KT88s) and even a jimi Hendrix reissue (the Super100JH uses KT66, as they found out that Hendrix actually toured with modified Superleads). Also very early amps (JTM45) used 6L6s, as those were based around US designs (Fender)