I would have agreed with you, if he was not believed, but it is not the case here. If you remember his earlier posts, you may wonder if he's sure about the improvement, or the opposite perceived by him.
In his post #166
and post #170
More often than not, people who added a powerful amp to their AVRs reported to hear more bass, hence warmer sound, but apparently not in his case.
Regardless, while his speakers are not designed to handle a lot of power, they are rated 4 ohms so the Monolith should help keep the Onkyo cool for loud level listening. So to me, unless he really felt it makes the sound a little harsh and fatiguing, then he should return it and keep the money, or try another amp.
The Onkyo RZ 900 is class AB as far as I can tell.