As the power tubes burn in their bias will drift. The bias needs to be checked and ajusted for the first hundred hours or so. Even with tubes purchased from ARC. Power tubes change as they break in. After the burn in period the bias should remain pretty steady.
Setting the bias for the power tubes on the VT 100 is not that hard. The driver tubes are a bit more tricky. I owned that amp for a number of years and set the bias many times.