There is no 20 minute warm up here. At home I turn it on and play it when it is ready. Live... I turn on my amp with more than enough time (2 - 3 minutes) to be ready when the stage is handed over to the band. Does my amp sound warmer about half way through a show? Maybe... but I'm more inclined to believe it's my ears getting acclimated to the surroundings. IMO - if an amp is changing drastically (something more than a slight nuance) over a 20 minute period... something is failing under stress and needs to be replaced!