Check the heater voltages. If they are above spec, the tubes will fade a lot quicker. Most of the time this is due to high wall voltage above 120V (or 240V) so you can't do much about that other than a transformer. If the wall power is okay then the amp should be serviced. Anything above 6.3 or 12.6 volts on the heaters is either careless designing or fading components. You don't want to stick expensive NOS tubes under those conditions.
