Worth mentioning again - a variac will lock the power at a fixed amount up or down from the wall current. If the wall current fluctuates, so will what comes out the variac. Set the variac to drop the wall current 7vac when it's at 125vac, and you're golden. Same drop setting if the wall current hits 120vac puts you into brown out territory, which can do as much (or more) damage as high levels.
I'd only use one if you already had reasonably steady power to begin with and just needed to tweak the range.
Thanx for the reminder though. My line level here was averaging 125vac, so I built a lil bucker to drop it to around 117vac. Power company came through and did something, and now the line level averages 120vac as it should. No longer need the bucker when it's like that, but I do like to check every now and then to see if anything's changed. Been real steady lately though, so maybe the worst is behind me.
I do like the idea of a good line conditioner, but I'm just too dang cheap to make the investment. The only critical component in my system is the ST120, and I can always build another one for the same money ...