I think there is a lot more going on than just the amplifiers so unless you know that is causing the difference you may be disappointed. Or not, bias can be great.
Setting gains in a chain can be complicated. If you set the output of the Onkyo too high it will distort the input of the HK. Too low and noise will be higher and you might not be able to drive the HK to maximum output. If you are using the Onkyo's processing then I would do as you said and adjust the HK so that the Onkyo's trim levels are near 0 dB, after verifying that the Onkyo's output won't overdrive the HK (probably be fine).
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley