easiest way to tell if voltage is affecting your sound, they sell the plug in wall monitors for under 10 bucks at Walmart. Just watch what it's doing, in relation to what you're hearing that day. You might be as surprised as I was.
I use a Brown Box(set step variac) after the Black Lion. I run consistently 117-118V on the BB display. The BL display stays pretty consistent between 119V-121V, and usually 120V from what I have observed. I get it though, for someone like you that is taking very precise measurements, you might...
Like @TheGreatGreen said, I'm not sure it's going to help with EMI. It definitely helps noisy mains though, I get very little noise in my amp room. After getting a black box, I saw my wall voltage could swing between 117-126 volts. You can hear and feel the difference with that much voltage...
I gotta disagree. I still hear Billy Idol tunes on random radio stations to this day, and my kids know his popular songs. I think Steve was a great writer, if you can name a song by the first riff, you have to be doing something right.