Those old boat anchors I used to call them such as the early Mrantz's and Sansui recievers no doubt had a lot less failures as a result of connecting 4 ohm speakers. I still have a 2270, and the thing is a tank. It also weighs about as much. But pop the hood and you will see a massive transformer, and some very beefy caps. It is also completely solid state. It was built a lot like many of the pro power amps.
Interestingly enough however, many of the newer models from Crown have employed innovative technology that ends up delivering high power at low impedances in a lightweight package. Surface mount technology plays a part, but I think there is more going on. Cooling and heat sinking is almost as important as performance, and unlike a home application, in the pro environment, the fan noise isn't a factor. The challenge in building home products is doing it without a fan involved