Beware of the variacs, many of them are autotransformers, so the output is not galvanically isolated.
At 500W, it can rather make sense to use a handheld oscilloscope, or to float the oscilloscope instead of floating the DUT (Device Under Test).
I made an improvised DYI isolation transformer out of two identical 220V/2x22V (they were meant for a stereo audio amplifier with independent power source for each channel) connected secondary to secondary, and the remaining primary windings were used as isolated input/output.
220V/2x22V -> 2x22V/220V
Since each transformer has symmetric 22V secondary winding, I can also connect the 22V secondary from the first transformer to the 44V of the second transformer, so in the end I can get an isolated 220V/110V, too.