I would say those big 8T towers aren't as difficult to drive than just the impedance graph alone might indicate, because of the good sensitivity, and you noted it even had a bump in the bass..., and, phase angle isn't horrible to that may help the average high power amps too.
From your linked review:
6500 cu.ft is big, but what's your listening distance, 4 meters, or 6 meters, or more?
Not saying anything bad about the STR amps, they are very powerful for sure, more powerful than the X350.5, for real world use for sure, when literally continuous ratings aren't as important, Anthem output specs for 2 ohm dips seems to be rated for up to 3 seconds, that's long enough. I would think real world contents rarely has bass frequencies the range shown in the impedance graph that would last longer than 3 seconds, except some rare huge pipe organ music. Again, amps like those are powerful, I am just being curious about the minor details, and, the typical marketing hype (such as "massive"
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) that usually exaggerates things to make their points.