Just got in the cheaper unit, the Brownie yesterday. Haven't put it through its paces yet. I run a variac set to 115vac at home for my vintage amps. I wanted something portable for when I took the amps on the road.
I also run a variac with a Mesa ta15. Like most el84 based amps it runs them hard. At 117vac compared to normal line for me at 124vac, I notice the amp runs cooler and sounds less strident. The biggest thing I notice with all the amps, is that by controlling the voltage my tone is much more consistent day to day.