Cam, it may be premature to be thinking about more cooling. If what you say in the first paragraph is correct and you've run it as loudly and for a longer period in the past, it indicates that the temperature explanation is less likely. My first thought would be in the second category, i.e., a possible short. Although you apparently changed nothing before it shut down, there'd be some possibility of a loose strand of speaker wire that made contact at some point because of vibrations from the sound. Even if it looks good it'd be worthwhile to very carefully reconnect the speaker wiring. A more remote possibility is that an internal short developed in one of the speakers because of the sound vibrations.
If the circuit in the amplifier that protects the speakers from being damaged by DC shut it down, there are a lot of parts in the amplifier that might have failed and allowed the DC offset, and this wouldn't be anything that can be remedied at home. You should turn it back on to see if it shuts down almost immediately(with program material playing, not just on standby), which would also indicate that it wasn't a temperature problem.
