we own a real Plexi (+ an XTC which as you might know has a Plexi mode, + a Marshall JMP1C – the small NMV 1W combo), and that’s for sure a “hot” topic. At least here it has already led to long discussions between me and Davide. Many people think a big part of a plexi tone comes from the poweramp, at very high volumes, and tamed in some cases by a Variac (EVH) and/or an attenuator. That is obviously true for some historical guitar players (EVH, YJM – at least at the beginning of their career), not for others and at least not recently (Gary Moore got his distortion mainly from pedals, the same could be said more recently for Paul Gilbert).
IMHO, the tone of a Plexi is done also by the preamp section, which can be kept under control with a PPIMV or a volume pot in the loop section (it’s present in the new SLP reissues).
One day we will probably approach the modeling of a poweramp section which can be cranked. For now, similar effects can be obtained, IMHO, by reducing the number of gain stages in the preamp section and by pushing them a lot – something very good in this way can be done on the normal/crunch channel of SaturnLO (decreasing the voltages is very useful).