You know how much the sound of an amplifier changes from cold to normal. Naturally, the change of sound does not stop at that temperature. When the amplifier is driven hard, the temperature rises even more, causing another change in sound. This is thermal distortion. The sound quality of the amplifier keeps on changing as heat levels rise and fall. We have reduced this problem by using a high idle current, ensuring the amplifier remains medium warm all the time. It draws about 140W at idle. The normal demands from music are only a tiny part of that, so the temperature remains fairly constant. The mass of the heat sinks also acts to stabilise the temperature. In a cheaper amplifier with too small heat sinks and perhaps 10W idle power, the thermal distortion is much worse. Even Class A amplifiers have thermal distortion; they actually become colder when you play louder.