Heat is a byproduct of any audio power amplifier. Class AB amplifiers typically waste 40% of the power they consume as heat (Class D designs less than 20% on average). The more power one makes at the loudspeaker, the more heat is generated that must be dissipated. No such thing as a free lunch. Heat is generated by the power supply and the output section. The power supply creates the most heat at WOT while the output section coasts (a fully opened valve does no work). Somewhere between WOT and OFF is the point at which the power supply and output section create the most combined heat - that is pretty widely accepted as 1/3 power (1/3 of the amplifiers rated power). Power is the product of current & voltage. If voltage stays the same and current doubles, so does power. Then, it's up to the heat sinks to dissipate the power which is wasted as heat. The McIntosh designs with autoformers can have less heat sink mass than comparably sized direct coupled designs with similar power ratings. The direct coupled design would need that additional mass when being used to drive low impedance loads whereas the McIntosh user would simply choose an appropriate tap.