Hello mazian. It is important not to operate a tube amp using output transformers without a load on the output terminals. (As opposed to solid state amps which don't care about having speakers connected.) The advice of others responding before me is quite useful. So if you decide to play music while putting hours on the tubes (not really necessary, as they won't sound "bad") substitute a suitable resistor for the speakers if you don't wish to hear whatever is playing. I encourage to play your favorite tunes and enjoy the music.
Simple question, probably stupid
Is it necessary to have sound (music or white noise) playing through the speakers when burning in new tubes? I have bought a matched quad of Shuguang KT88 Z Black Treasure tubes to compare with my current Gold Lion KT88 and Tung-Sol KT120 tubes in a Rogue Stereo 90 amp. Reviews of the Shuguang tubes indicate that they need around 200 hours of burn in to reach their true potential but I am not looking forward to having to listen to 200 hours of crap sound.