I also have a Denon 3300, the 3300 have a reputation to overheat very easily.
Tho, I had play it for long period of times at very high levels, 5channels (-13 to 5 db) with my M60 and a couple of JBLs. Luckily that never ever had happens to me so far. I guess is because I have it sit just on the tiles floor with nothing on top and neither side of it. I had touched the sinks, it gets hot but not unusually hot.
Probably yours is defective 'cus -28db is way low, unless you really have it in a place where it cannot breath from anywhere.
