Sed, welcome. Your initial question seems to be based on an assumption that the Axioms would require more power than the Energys you use now. If anything, the contrary is the case. The C-500 and M80 both have a 91dB sensitivity spec for 1 watt input, but Energy bases their numbers on two speakers operating, which boosts the figure by 3dB. So, on a equal basis the M80 is more sensitive and requires less power for a given sound level. The same point applies to an even greater extent to the center speakers; the little C-100 is significantly less sensitive than the big VP180 and requires more power. Bigger speaker doesn't necessarily mean bigger amplifier: the bigger speaker is often more efficient and uses less power.
Your 6160 should have more than enough headroom for the Axioms. Once you have enough headroom for the levels involved, any more is simply unused and is meaningless. Speakers don't acquire human traits and it isn't helpful in what should be a serious discussion of audio technology to casually throw in an anthropomorphic comment about how they "love" higher power ratings.
As to the impedance setting, under no circumstances follow the direction in the manual to use the lower setting. This is a legally required warning to help prevent overheating, but it does this by reducing the voltage output of the amplifier power supply section. Obeying the law(Ohm's Law), this results in lower current and lower power capabilities. Tests show this cuts maximum power roughly in half. Your Axioms will do fine by staying at the default 8 ohm setting.
