Crappy design can also cause an amp to go poof. I remember a local shop telling me that 25% of their Soundstream Reference S series blew caps upon first power up! That is what gave them the nickname of Smokestream back in the day.
Also, the Orion HCCA G5 series had an approximate 50% failure rate and was notorious for catching on fire when it did go poof.
I just wanted to throw that out there because not all amplifiers blowing is a result of improper installation.