I have purchased 100 watt 8ohm dummy loads from Parts Express just for this purpose. Pretty cost effective at $13.50 a pair. They work great and they genereate no sound although they can generate some heat. Easy to keep those tube amps warmed up without any speakers or damage. Check it out at: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=019-020
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?