What Al said. Most tubes are cheap compared to the price of good output transformers.
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?