This is a relatively common problem on Onkyos (so I've heard, HD Guys podcast and other sources), I had the 607 model but always have used an A/V rack made of perforated metal, specifically it was an older Bell O'getti stand that happened to look nice and had perforated metal shelves. Also if I have any components that don't have more than 1/8" clearance from the bottom of shelf due to having small "feet" I will put thicker bumpers on them to raise them to 1/3"-1/2" off the bottom of shelf.
I'm really anal about temps having lost some good equipment over the years but ever since I've been doing an open, perforated shelf rack design, and raising the feet on low clearance components, I've had zero problems.
If you have a Laser IR temp gun, aim it inside your Onkyo sometime when it's running, you'll see how hot it gets above room temp. I also have used some wireless Honeywell temp sensors that you can put in a few places amongst your rack and they report back to a bigger LCD screen (about $50 on AMZN, handy for lots of things. . .) and you can keep track on the ambient air temp, but remember the internals will always be much warmer.
Hope this helps, let us know. . .and congrats on getting Onkyo to foot the bill, that's good to know they stand by the customer like that.