What I have found with both Valve Art and EH KT88s in my ASL Hurricanes is that they take a couple of days to stabilize bias current in a push pull amp. In that two day period, the bias fluctuated as the tubes burned in. This you need to watch closely for a few days before business as usual. I have noticed zero change in sound if kept biased within tolerance. Those who say they "warm up" are probably experiencing ear burn in rather than tube burn in - a phenomenon that makes us think it is getting better, but we are actually training ourselves or becoming acclimated to the new sound.
