@yoyoyaya As you may know, the efficiency was used to be measured based off a single tone at 1khz via industrial standard. Multi-tone test based efficiency was available through individual sources though. You are right, even looking at the raw impedance chart, the impedance drops down to 1.75 ohm at approximately 2.5khz. JA also commented:
"While this will not present pedigree solid-state amplifiers with any drive problems, tube designs are best avoided given the overall low impedance above 150Hz."
@dishman442 Thank you for the Odyssey mono recommendation. I will look into it.