Well, I have been building speakers and amps for 40 years, so I rest on my experience.
I am looking at objective measures. Always first. I then listen, but invariably, my ears usually follow the specs until you get to a "good enough" threshold. Loads have HUGE differences on ampler performance. If you have ever done a Spice model of an amp with various realistic loads, you would see it in the FFT big time. Now, before the "good enough" everyone has their choice of what faults are least objectionable.
One of the things I do not understand yet in ClassD is how distortion varies with current. AB is usually far worse with a "4 Ohm nominal" load than an 8. Sometimes by an order of magnitude.
Super low output Z is no holy grail. If you have ever built a speaker you would know it is darn near BS.
The data sheet for the 1ET400A shows output power with respect to distortion, which should say what needs to be said about how distortion varies with current. The module is noise-constrained until an output power of 100 watts or so at 8 ohms (200 watts at 4 ohms, and about 150 watts at 2 ohms) after that, distortion becomes the dominant feature until clipping. The point where clipping becomes significant is an opinion, of course. Distortion starts increasing significantly at 130 watts (8 ohms), 260 watts (4 ohms), and over 500 watts (2 ohms--500 watts is where they stopped the graph). Once the effects of clipping reach -45 dB SINAD, limiting kicks in for 4 and 8 ohms (500 watts isn't high enough to show this point at 2 ohms). Power levels at that point are about 185 watts into 8 ohms and 370 watts into 4 ohms.
Taking the noise limit, at 8 ohms the amp is moving 3.5 amps, 7 amps at 4 ohms, and a little over 8.5 amps at 2 ohms. Taking the knee of the curve, the current at 8 ohms is 4 amps, 8 amps at 4 ohms, and 16 amps at 2 ohms. Not much load effect up to that point. Distortion+noise at all of these points is less than -115 dB FS, which is astoundingly good.
At "hard clipping" (which is still less than 1% THD+N), the current is a bit less than 5 amps at 8 ohms, a bit less than 10 amps at 4 ohms. From my read of their data, all these points are voltage-constrained. Current constraint comes in a 2 ohms, and if I'm reading their charts correctly, it happens at 47 volts RMS, or 23.5 amps. Their spec states the amp will go into overcurrent protection at 25 amps, so my read of their charts is in the ballpark. That point is also better than 1% THD. 23.5 amps at 47 volts is...a lot of power.
This is the final amp, of course, and not the buffer. The amp provides 12.8 dB of voltage gain, but will happily amplify whatever voltage upstream gain stages present to it. That won't be a function of its own current capability, though.
Rick "suggesting close study of the datasheets for the Purify and Hypex modules--they are well-documented and I don't have 40 years of experience designing amps" Denney