Can a variac be used to maintain constant voltage?

I stand corrected! The power supply I used at home(test purposes), would only maintain, or maintain at at a reduced voltage. The veriac(Ohmite) I had on the job could up to double the voltages, presented to it(but still directly proportional to the input). Gotta stop the early morning posting.

My own reason for having this variac in the first place is that prior to the Threshold, I was using a pair of vintage vacuum tube monoblocks running 8 el34's per side: when turned on the amps would spike to the maximum level as gauged by their ammeters and then settle back down to the nominal 6 amp running rate: so, rather than having to deal with this initial surge and its potential negative impact, I bought the variac to slowly bring up the amps to running voltage. The variac is selling for a few dollars short of $600.00 now and I do not if its worth keeping this expensive piece for any valuable audio uses -- i.e. reduction of high frequency noise as Almarg mentions above -- or sell it off to someone with a more suitable application. I still find it amazing that the variac is showing a consistent 120 volts as set by my original dial setting. I had a PS Audio Power Plant Premier at one time and the incoming voltage showed as high as 125 volts sometimes. So it makes me wonder if the variac is not in fact more useful in my current Threshold application as might otherwise seem.

With your voltage only varying by around 1-2 volts, plug your amp right into the outlet. I would love to have my voltage stay that close to 120 volts. Mine will vary up to five volts, and sometimes more, the same same day. I live in a straight residential area. In other words, no commercial equipment running off of my part of the substation feeding the power to our house. My friends that even live in a different state have similar variations.

One problem with a variac can be when your amp draws a lot of current (when it puts out a lot of watts), the voltage will drop a lot more through the variac. Another problem a regular conventional variac will usually have a contact (sometimes with a motor type brush), that can add noise to your power. With voltage that stable, plug your equipment in without the variac. You will be way better without it.

I have a couple of variacs. One of them will put out about 130 volts, the other one will put out about 140 volts (if memory is correct) with 120 volts in. More voltage output yet, with more than 120 volts in. With my voltage running low, and a variac boosting it, it could output way too much when the voltage would go back to 120 volts, or worse yet, a lot more when my voltage goes to around 126 volts, as it does sometimes. That would probably damage something. Another good reason not to run one.

Variacs are mostly used for testing. One would be to correct the voltage to 120 volts for testing and measuring, another reason is to simulate what some equipment will perform like on 114 volts to 127 volts. This is what an outlet can give out, and still be within power guidelines set by government regulations. They can be used for testing and simulating, or temporarily correcting other voltage swings too. But still, they are mainly used for testing.