vacuum tube amplifier reflection sensitivity?

[CopperCone]

Is damage from reflections a 'averaged' thermal phenomena? Or does it relate to spot heating capable of doing damage much under the 'rating' of the device? (this is for traveling wave tubes)

Can you run them at a low power level (say 70% of rating) and just have them absorb the reflections without damage? (say its 20% reflection)

Or is the heating incurred by reflective absorption much greater then the heating caused by efficient generation of energy that is properly transmitted into the load?