I created many good presets using the VARIAC in Guitar Rig 3, but later I replaced it with the Lead 800 just for fun and I felt that it had a more natural response.
How I mentioned before Eddie was after the tone he heard inside his head, so I am not so convinced that he was only using his VARIAC amp. Many guitar player were using multi amp setups already. If it would help him in the studio, I can image that he would fire up a another amp, even a JCM 800 on the side and mic it up to make it sound better. I know this is just an idea, but I just want you to realize that are no strict rules, He didn't' have rules, why would you?
The other very important about the amp and EQ-ing, that you have to hear how the amp sounds in the chain, even if it sounds bad itself, it could be really good how it reacts with the rest of the rig (pedals, mics, effects, etc.)
The bottom of this post you can hear how this amp sounds like itself with the default cabinet. Its not really good tone, but it will sound good. Don't forget those setting are for the whole chain, not for the amp itself.

This is what the marshall jcm 800 sounds like with this settings. Notice the boominess
I set to VARIAC to 90 on the amp. Left the SAG alone. Little more RESPONSE and turned the BIAS all the way up, which cleared out the tone.
The amp comes with a Matched Cabinet: Lead 800 (4x12) but I will chang it later