> AC has gone down to 108VAC
This kinda crap happens. Which is why I hate to see "48V regulators" with nominal 53V input. Sure it works when the line is 120VAC, but it gets goofy at 118VAC, and lame when the backhoe meets the power cable and the patch-around only delivers 108VAC (as my city has been for parts of 2 years). A "regulated" supply that needs the regulator to filter the ripple will be NASTY when the regulator punks-out for lack of excess voltage to waste. Regulation means starting with LOTS of excess voltage, like 1.5X desired voltage, so line-dip and ripple and other crapola never lets the regulator drop-out.
As I'm sure you understand, a manual Variac set to crank 108V to 120V will be BIG trouble when the utility power comes back to normal: 133VAC!!!
Auto-variacs exist, unfortunately. One theater here had one that bobbled 115V-117V-115V-117V-115V-117V all day long. Such devices are widely used by utility companies, though most are better adjusted than this one was. Fortunately, they don't make one you can afford.
Constant-voltage transformers exist. The cheapest model puts out a round-square wave, good for lamps but not for DC toys. Most now are waveform corrected and make clean sine-like waves. They have a minimum load, they run HOT even when unloaded, they are HEAVY, and cost a lot.
Some computer UPSes will correct voltage. The very cheapest don't, unless it fails. Another class will make minor trims, using the inverter to boost/buck the line. Only expensive ones do this clean enough to use around audio.
Put on your mittens, wax your sled, and go play in the snow.