I have had many tube power amps and never had one that didn't sound great after 10-15 minutes. If the amp you brought home had not been played in a long time it may need more breakin. My David Berning ZH-270 tube amps sound perfect right from a cold start. Mt=y friend has an all VAC tube sytem which needs maybe 5-10 minutes warmup although it still sounds stunning from a cold start. The previous response which said what you found is typical is nonsence
Alan
Alan