Generally unless you have widely fluctuating voltage, you do not need a voltage regulator.I agree in general with the exception I've personally noticed with my tube integrated. When the ac voltage varies the biasing on the tubes varies in a direct relationship.
Normal voltage ranges of 115 to 125 volts are no big deal.
Once you are getting sags down to 110V or overvoltage of 130 volts Then you have a problem
