Heat in some of the older class-a designs radiates throughout the entire chassis, thus impacting all parts inside.
The old class-a solid state theory of leaving the amps "on all the time", comes at a price. Had to scrap some great amps "for parts" where certain boards and pieces inside could not be fixed or rare parts inside were not available any longer. Argh.
Some of my current mono tube amplifiers run much cooler than former designs, and thankful to designers for re-thinking how to make things last longer too.