There is nothing on those amplifiers which you could call a heatsink. Just the metal case. I wouldn't want to run them at 100 watts for too long!
Steve.
For <100W a moderate sized metal chassis/case "could" supply sufficient surface area to dissipate heat from. If the chassis is made from the typical steel, that will provide lower heat transfer so not work as well as a full aluminum chassis (an aluminum heat spreader could help some). The familiar finned aluminum heat sink gets the extra surface area from all the fins, and in top of the fins, if you look closely there are small grooves in the surface of the fins, this grooved surface increases the total surface area even more.
While large enough chassis could dissipate much of a small amplifier's heat, using a compact "real" heat sink allows you to design a standard power module that could be used in multiple SKUs, instead of every product requiring a different new heat sink design.
JR