Both receivers are rated at 140W/channel @ 8 ohms, except that the Pioneer is rated at that power with all 7 channels driven, while the Onkyo is rated with only 2 channels driven.
It may very well be that the Onkyo has higher current output capability, but without either manufacturer disclosing that specification, I would have to at least partially attribute the difference in heat output to Class D versus Class A/B.
Of course, Mr. Peabody's advice to audition them is good, but that probably isn't a possibility for you if you're wanting to buy mail order or over the Internet to save money and don't care about a warranty (in the case of the Pioneer).
Maybe if you post what these "power hungry" speakers are, someone may have tried them with one of these (or very similar model) receivers.
Steve