I believe that when a tube amp is at idle, it draws full rated power/current. Increasing volume doesn't draw more than its rated power...however, it can drive the amp to clipping, which can be detrimental to the life of the tubes..especially tubes that are worn down, and don't hold their bias.
A question I suspect reveals a lot of ignorance...
Does the volume at which I listen affect the rate at which my tubes "age"? I'm thinking more volume requires more voltage and higher output voltage levels represent "wear" on the power tubes. Thanks in advance for your tolerance.