Neither of these amps is underpowered, if memory (and a little research) is to be believed, the HK is a 65w, and the Onkyo is 70w.
This is enough power to drive any Axioms. It will depend more on the volume level you desire and the size of the room you are attempting to use. The room size is more important to the equation, especially if you are trying to "fill" the room, or provide ambience or background music.
I drive a pair of large towers with a 40w NAD...and I drive them hard on occasion, with no complaints. The room is about 12x20.
Regards,
BBIBH
