If cranked gain Plexi tone is your destination then series I is my preference, it clanks and chirps like a cranked, loaded down plexi, I have a 68 metro plexi and that's what it sounds like, variaced and loaded down. I've always thought your RT mod series I has gotten the closest.
The series II sounds more in line with a 2204 style of gain response IMO, it sounds good, just different just not exactly what I hear in a great plexi tone.