The relevant bit of information regarding my first point is the deviation of the dark gray line from each other when comparing both graphs. After all, you're not switching from 8 ohm speakers to 2 ohm speakers when trying to compare these amps, you're using the same speakers.
Regarding your point, besides some curves in these graphs looking like the were manually inserted (though this might be due to lossy compression), I don't know how they measured the output impedance.
If from 8 to 2 ohms there's a 0.75 dB drop then the (effective) output impedance is about 0.25 Ohm.
For a 0.25 dB drop it is about 0.08 Ohm.
The black lines relate to your point on output impedance.
The drop in output at 4 Ohms and 2 Ohms shows the amp with lower output impedance having a greater drop.
Since most speakers are reactive loads, the effective load placed on the amplifier can have an effect. It may even trigger limiting when the voltage.
Nothing in the output impedance measurements of the above amplifiers explains the drop in output at 2 Ohms.
In fact, the frequency variation in the black line is far less that the load.
There is also no reason to believe that a combination of load and phase could not produce greater deviations in linearity.
This is an interesting article:
Heavy Load: How Loudspeakers Torture Amplifiers
Why, in loudspeaker reviews, is impedance measured (assuming that the magazine in question bothers to measure anything)? Generally, for one principal reason only: to establish whether the speaker presents an "easy" or a "difficult" load to its partnering amplifier. In the design context, much...
www.stereophile.com
Output impedance influences linearity with resistive loads but EPDR can be used to get a better understanding of the load presented by a speaker.
EPDR is simply the resistive load that would give rise to the same peak device dissipation as the speaker itself. Adopting the EPDR view, the red traces of figs.5–7 become those of figs.8–10.
Taken together, these figures confirm that the orders of EPDR identified in figs. 8–10 are of real, practical significance when playing music signals: speakers really can make these high demands of amplifier output-device dissipation in normal use. If the amplifier's protection is invoked as a result, then its output will be clipped, even though the speaker's voltage and current demands may be within its capability.
This article is useful if you want to understand why the B&W 802 "has a reputation for being an amplifier ball-breaker."
Output impedance of an amplifier is an important metric, but is insufficient to describe an amplifiers performance driving reactive loads.
With respect to amplifiers, we have objectivists that can read these charts, see that even with simple tests, deviations can delve into the audible range so must rely on ABX to explain why it does not matter. I'd expect a call for more and better measurements.
- Rich