I will plead guilty to expressing myself lazily for the sake of brevity. What I was getting at is power factor and heat dissipation in the output devices. This can not be calculated from the impedance curve alone. The phase angle must be taken into account. This is a major factor in why an amp driving a real speaker is so different from driving a resistor on the test bench. This is just one of the reasons why I like my huge tripled output devices in my 909s versus puny receiver output devices.
It is also the reason I design speakers for easier, rather then difficult drive, and will sometimes add components to correct difficult drives, like Raymond Cooke used to do.
This article is a slightly difficult read but it explains well a topic that needs to be understood, and why certain speaker amp combinations can be lethal to amps. It also explains how this issue can blow up bi-polar, MOS FET and class D output stages in different ways.
The bottom line is that the more robust the amp you purchase the less trouble you are likely to have. I have done well with my current dumpers and so I stick with them.
