Heat

You are right about heat being generated internally. From there it flows into other things such as the heat sinks by a process of conduction. When you put a spoon in a cup of hot tea and the handle starts to get warm this is conduction. So the amp heats the heat sinks and this takes some of the internal heat away. But what if the outside temperature is higher? Then less heat flows out. That's conduction.

Now on the outside some of the heat simply radiates away. If you've ever seen infra-red images that is from this radiation. Some heat also warms the air. Warm air rises being replaced by fresh cool air. This is convection. This is why heat sinks are vertical, to maximize convection. 

Convection can be increased with a cooling fan. The more air the more convection and the greater the cooling. Always depending of course on the temperature of the air. A heat gun is after all blowing heated air. 

So yes your amp will get hot and run hotter in a hot attic. Most all SS amps have thermal cutoff for heat protection. It is best to avoid this. If it does happen however simply let it cool and resume. Then look for a better cooler location for your amp.

Thanks millercarbon, that's a great explanation!

Heat is the enemy of audio equipment. The lower and more constant you can keep equipment, the longer it usually lasts. Failures primarily occur from thermal cycling. I'm not saying to keep your equipment on all the time and I don't like the noise a fan makes. But when you listen at moderately loud to loud volumes, a fan will help extend the life. I use Rotron whisper fans.