I was sharing Studio America in Pasadena with VH while they were demoing their second LP way back when, and the Variac was definitely down, not up (how far down varied he said, but typically around 92V'ish for recording, live more or less depending on the room). When I first heard EVH warming up out in the studio, I ran out from the control room to meet him as he was warming up with Eruption, and I'd never heard anyone play anything like that at that time.
EVH was a real nice guy then, showed me his rig etc, and after my jaw got off the floor from watching him play, we hung out a little before the rest of VH showed up and they went to work. We spent a few days sharing that studio, us in the afternoon, VH at night, and all the guys were very cool except for DLR who was of course an egomaniac (their first album was out and had quite a buzz already, but they weren't full blown stars quite yet as in those days you needed a successful follow up ASAP).
Note that the magic plexi was dimed and louder the all freakin' heck, even browned out! When EVH fired the magic plexi up through a single cab 4X12 out in the studio, it came right through the doubled-sound proofed studio walls and into the control room in spite of all the sound proofing and double-double glass LOL! Note also that he had a small piece of plywood with a Roland 10-Band EQ (which I believe he used as a clean boost with his Strats, and as a clean boost with EQ for his Explorer?), Flanger, Phaser, plus an Echoplex on the floor near the amp at that time, and he sounded just like the recordings in that room with not much done to his tone at the console except for some minimal EQ/Filtering, the panned reverb, and of course mic(s) and placement(s).
Crazy loud they were in that small studio...
Note also that the vintage Marshall Output Transformers of the day were quite on the edge of flaming out under normal full-power use, so upping the B+ would be a sure fire way to create fireworks (flamed some myself back in the day without a Variac, with glorious eruptions of PCB's LOL)!