Corazon, I'm no expert on tube amps, but AFAIK you should not operate it without load. The fact that you don't apply signal doesn't change anything since practically anything can cause output transient that, with open transformer, will generate high voltage (called flyback voltage) that can fry output transformer (breaking insulation). Even switching power on or off can do it.
As for using Variac - 1.5 years is not enough to bother IMHO. I would be concerned with something that was unpowered over three years. Without voltage electrolyte eats up aluminum oxide layer reducing capacitor's breakdown voltage. When voltage is present aluminum oxide is being rebuild.
As for using Variac - 1.5 years is not enough to bother IMHO. I would be concerned with something that was unpowered over three years. Without voltage electrolyte eats up aluminum oxide layer reducing capacitor's breakdown voltage. When voltage is present aluminum oxide is being rebuild.