Most of the changes are a simple component value changes
As for the VH tone in the plexi Mk II that's more or less they way the amp tone controls and gain master volume are set there are way more settings to get different sound out of that amp more "classic" for a lack of a better term.
My amp I altered things to my liking. Tmb side is more of a cross between plexi 1 and mk 2 with a few other things tossed in. And the normal side being in between plexi 1 and 2 just changed a capacitor value
This is with the amp wound up but turn things back down and your listening to 70's show sound track
http://soundclick.com/share.cfm?id=12242475Like COCO said the board's are the same its just the choice of tweeking values. The only real different one is with the v6 and its just the extra components to switch between el84 or 6v6 output tubes. The preamp can be any one or part of the others as well as other twists thrown in ! TMB plx1 plx2 sIII
Sorry bad link to Unimind's clip thought it was a dialed back one
Last question I know you said you were not interested in VH tone but what are you looking for in tone An idea of what you want might help. crunch , High gain , clean. bluesy . What sort of guitar strat , les paul. singe coil double coil.