Onkyo receivers have serious problems with over heating.
I highly recommend to you, to do something like this.
This cost me 0€/$ and works perfect!
And what is so special on this project? Fans go ON & OFF with receiver and they are inside of case ;)!!!
You must disable HDMI Thru!
I found this cables with 12V on HDMI motherboard.
For protect connections I used Sugru.
I had to remove PC/VGA connector, because there is not enough space for another fan.
A little bit modified fan holder;)
Both fans are connected to new 12V wires.
The left fan is original and blows up/out.
The right fan I add and blows down.
They are a little bit noisy. But unheard when I listen music or watch movies.
Some day I would maybe upgrade with 4 slim and quite fans.