I have always found that tubes do get better with "burn in time". As others have said and as Brent Jesse who is my go to for NOS tubes says you have 2 options: leave the amps on for the desired period of time, no signal needed or just burn them in by using them. I've generally done the latter as I really enjoy hearing how they change.
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.