Nikon, I understand you're not inclined to listen to what you perceive as non-experts on the internet, but understand that impedance curves are just that--curves. So at any given time, two speakers on different channels, even if they have the same nominal impedance of say 8 ohms may well be presenting a different impedance to the amplifiers in the receiver. Note "amplifiers" plural.
I would wager that very few people have 4 ohm speakers on all 5/7/9/11 channels. And I have never once heard of a receiver or speakers being damaged by mixing impedance. Many people whose knowledge on the subject I respect (including Axiom employees) have said categorically that there is no problem. If you're not comfortable with what you're hearing from Onkyo, return it. Or buy M80s for all channels. Or M60s instead of M80s.
