Vacuum tubes need to come to thermal stability, that is reach a stable operating temperature. That is one criteria for their operating stability. This is sometimes difficult to achieve especially if the equipment is used in an unconditioned environment such as in an unheated manufacturing plant or in places where they might be exposed at times to an external source of heat such as a furnace. Being located near a baseboard radiator in a home might not make much difference. Sitting out in the open as opposed to being placed in a closed cabinet with little ventillation on the other hand might and might even shorten its lifespan. Clearly the voltage applied to the filaments and therefore the current they draw will affect the amount of heat generated and therefore the operating temperature. Most designs do not regulate this voltage especially if the filaments are heated by ac directly off the power transformer. Remember that the wattage is proportional to the square of the voltage (Vsquared/R) so a small change in voltage can create a large change in temperature.

The effective solution is the one most electrical engineers embrace and many vacuum tube loving audiophiles types reject, and that of course is negative feedback. Negative feedback stabalized gain, reduces distortion, and extends frequency response. Changes in operating voltage become much less important. And of course, it takes feedback in the power supply to regulate it and reduce voltage fluctuations further stablizing operating points. In the very early days when vacuum tubes had very little gain, say 20 and were not very reliable, negative feedback hadn't even been invented yet and would have been impractical if it had. However, when tubes were manufactured with gains of 50 or so and became much more reliable and cheaper, negative feedback became a viable design option. Until relatively recently, very few electrical engineers would have considered designing an amplifier without it. But in our modern try anything era, some people have taken a giant step backwards.