All the speakers you are using like most other manufacturers are standard 8 ohm spec. so it is not as if they are M80s or the other large models that spec. down to lower resistance. Sounds a little strange and perhaps you are playing them VERY loudly over an extended period so just maybe it's time for an outboard power amp. Even though I am using a Pre-Pro with separate power amps even prior to this with AVR's at high volumes, I have never encountered the shutdown issue you have experienced.