Tube brightness and bias

I have a pair of VTL Deluxe Mono 120 and I notice that one of them have its tubes lit up quite a bit brighter than the other although all the tubes have the same brand name. It also comes on a lot faster, about 4 or 5 sec, than the other. If I turn on the preamp first, then turn on both amps at the same time, then music from the amp with bright tubes comes on about 4 or 5 sec before the other.

I don't have much experience with tubes but I wonder if this has anything to do with the tubes being either over or under biased. If I reduce the bias, would the brightness then reduced?