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On older Marshalls there were several changes made over the years to filter cap values. One arrangement that they used in the late 60s was two 100uF caps in series for the plates but then two 32uF in series for the screens...then I think it was two 50uFs in parallel for the PI. In any case, there was more filtering on the power tube plate node and the PI node than on the screen node (which was only 16uF). Since these are often considered to be some of the best sounding Marshalls...I was wondering what effect having low-ish filtering on the screens and high-ish filtering on the PI has?