As a general rule do not have a tube amp on without a load (speakers) attached. There may be a few tube amps that this does not hold for, but don't find out by mistake that yours is not one of them.
Warming-Up Amps w/o Inputs or Speakers Attached
How dangerous is it to warm-up amps without inputs and speakers connected? When running A/B comparisions between gear it's difficult to truly gauge differences when one piece is good and hot and the other stone cold. What about the differences between doing this with SS vs tube amps? Can you just plug them in and let them cook?