I have tried a couple of modded Plexi's and 800s - and sure if you want mick Mars tone, its in there - but I much prefer these amps with a Pedal pushing them.
If you really want the "modded" sound, I think Bogners, Riveras, etc record, and play much better and are more toneful. I think the smaller companies got the idea right.
Else, try the new JVM, Vintage Modern, or Marshall 6100 and Jubilee - from what I know from techs - the circuits were tweaked just right to make them sound good with high gain - ie the Jubilee has a cleaner tone stack and then diode clipping. I don't think you can just slap an extra tube in those amps and get the tone.
If you really want that vibe, do consider the Soldano Hot Mod, it works in an instant, and if you hate it, take it out! total time to install AND un-install = 4 seconds.