Denon / Axiom is a very nice combo. I have a low end Denon and it powers the Epic80.
However, it will also get hot. All receivers will get hot, especially if you crank up the volume. Just how loud are you playing them? Do you have a big room? You may be dissapointed if you switch receivers and notice it is hot after cranking the volume up.
I see your Onkyo is rated at 75wpc. If you get a receiver with more power, it will likely get hotter. If you get one with less, you won't be able to crank the volume as much.
Is there another reason, other than heat, to replace the unit?