Hi Fred,
That graph is from the National Research Council lab in Ottawa, where Soundstage.com does many of its speaker measurements. If you go to Soundstage.com and then access their measurements, you can scroll down to the impedance vs. frequency charts for lots of speakers they've tested, including the M80s.
Yes, the Emotiva amps do have issues with low impedance high-current loads, as do some other manufacturers. Often a two-channel amp will do much better than a multi-channel amp, largely because the multichannel versions can't handle the heat issues. One Axiom owner had five M80s and went through a number of multichannel amplifiers, all of which would shut down driving the five M80s to extremely loud levels (those were Outlaw and Parasound multichannel amps). He settled on a two-channel amp for the mains and a separate three-channel amp, if memory serves.
Regards,
Alan
