The signal response measurements are done at 1kHz.Most ARC amps have lower effective output impedances than the majority of tube amps, and the VT100 is no exception to that. Specifically, it has a specified damping factor of 11, which for the 4 ohm tap corresponds to an output impedance of approximately 4/11 = 0.36 ohms. Per JA’s measurements the minimum impedance at any frequency of the earlier version of the Sophia is 3.26 ohms, with a near-zero phase angle (i.e., an impedance that is almost purely resistive) at that frequency. And at nearly all other frequencies the impedance is significantly higher.
As you’ll probably realize, the result of the interaction of that kind of amplifier output impedance with those speaker impedance characteristics will be not much different than would occur with a solid-state amplifier, namely very little variation of output voltage as a function of frequency. And yes, in a narrow range of frequencies in the area of about 40 to 50 Hz, where the speaker’s impedance rises to around 14 ohms (assuming the speaker’s impedance characteristics are similar to those of the earlier version of the Sophia), and also in the upper treble region where the impedance also rises significantly, the amplifier’s power capability will therefore probably be significantly less than at 1 kHz. As a result of limitations that presumably would occur in output voltage, however, not as a result of limitations in output current as you had stated.
However in the rest of the deep bass region, as well as in the mid-bass and mid-range regions, which encompasses the regions where most music contains most of its energy, the 1 kHz power capability measurements can be expected to be essentially applicable.
Regards,
-- Al