I have an old (1980) ISKRA Variac that is still operating perfectly.
I use it for the maintenance and for the first test of power amplifier, where a reversed big electrolytic could be very dangerous if reversed connected and powered from direct ac mains.
I think I cannot do any audio work without it..
When buying it, please consider that most of the times you'll use it at full power only when it will be in "unity ratio" position (e.g. output voltage equals input voltage), and in this condition it can delivery more power than it's nominal value.
I use a 2.5 amps Variac (575 VA at 230 V ac) to power big amplifiers that draw more than 1.5 kVA, and the Variac works well, without saturation and sagging of the ac waveform.
Obviously you should not drive the amp when there is a big input to output ratio of ac voltage.
Put the variac inside a vented cabinet, with a panel meter showing output voltage, an ac outlet, a time delayed fuse (or better a resettable thermal switch) and a dual circuit switch on the input side, switching both live and neutral.
See the attached photo of my "dinosaur". built in 1980, and never finished...
The Fixed Output was not a good idea, i do not suggest to make previsions for it: it's confusing.
regards