Any power regulator will do the job you need done, some of course do a better job than others. If you have an under voltage, it's going to make the amp pretty dirty sounding, which is why some players purposely use a Variac to do that very thing. Using a UPS is one way of compensating for low and high voltage issues, though I don't think I'd want to lug around a battery, and that's how a UPS works, it will clamp down on a high voltage, and use the battery to compensate for an under voltage. I'd just use a Furman P-1800 AR or something like it because it will regenerate the correct voltage internally without a battery, and it weighs a lot less than a good UPS. Also, no battery to fail during a show. There are boxes that will reduce voltage, like the Brown Box, but that does you no good if the voltage is already too low. The Furman box is expensive, but it will serve most players faithfully for a long time. I use a Furman power unit here in my studio.