And that's how I've set-up my system, with the 1500 driving the fronts and center, and the 805 driving the surrounds. Really works great. No problem with sounds panning from surrounds to fronts, that I can tell. And the over-all effect is MUCH more, how to say this, engrossing? enveloping? Its less like you're listening to the movie and more like you're in the middle of the movie, if that makes sense.
And I agree, voltage=voltage, shouldn't make any difference if it comes from an Onyko receiver or an Axiom amp, right? Only thing I can figure is, even at moderate volumes, the power required to fill my big great room (10,000+ cubic ft?) with sound is way more that the Onkyo can cover at all times, and thus distorts to a certain degree. While the ADA1500 has no problem, with room to spare, even at high volume (which I've experienced btw, and its just wicked cool).
I really feel like, for the first time, I've stepped completely into this mystical "Audiophile" world. Cool place to be.

M80v2, VP150v2, QS4v2, Onkyo 805, ADA1500-3, Sonos