With these new conversion boards , the manufacturers recommend using 4 ohm speakers , instead of the factory original 8 ohms .
Although I do run 4 big 8 ohm house speakers for extended periods of time when doing the bench tests before sending back to the customer,
and I have never had an overheating problem with the AAR boards . The same may not be true wit the Aroura boards I don't know .
Also the amount of vol will of course increase the amount of heat over time.
I would look at the impedance of your speakers next , and see what you have.

Greg