Do you have the Marshall pack for Amplitube? If so, try the tube screamer in front of the red one --- or actually, they have a JMP-1 model in there as well - Gibbons had used that for many years. If you have the JMP, try it without the tube screamer, because you can get alot of thick gain out of it.

The Plexi isn't a model per se, it's sort of a nickname for earlier Marshall amps (late 60s, early 70s). Most Marshalls have the same character with varying degrees of preamp gain, so try any Marshall and mess around with the preamp gain, with or without the tube screamer in front to see what you can get.

Happy Tone Chasing!

Mike