That's because system 'Qr' which is what this is all about is measured at resonate frequency where the reactances cancle out.
If you're looking at a simple resistance (representing output impedance of the amp at a given freq) driving a reactive network, there is no "cancelling out" of the reactance. Here's an interesting article I found regarding speaker reactance:
https://hometheaterhifi.com/technical/technical-reviews/a-secrets-technical-article/Zero output impedance is ideal, although unattainable, so the lower the better, but where does it stop making a difference? I like to look at it as orders of magnitude, and two orders of magnitude (minimum, as a rule of thumb concerning impedance ratios) gives us a damping factor of 100. This is only attainable with adequately low speaker wire resistance. Note that some speakers dip very low impedance wise (under 2 ohms, like some ESLs, and Gallo speakers, those with transformers are notorious for this). However, this static impedance number only tells half the story, and is generally an over-simplification. There is a phase angle from the back-EMF of the speaker, and this can amount to a much worse situation than driving a simple 2 ohm resistor. Ever wonder why some amplifiers "can't handle" low impedance speakers? This is why some manufacturers say stuff like "stable down to 4 ohms" or "speakers less than 4 ohms are not recommended".
It's like a hockey fight ---- harder to land a solid punch when you're sliding on the ice. The slippery ice is like high output impedance! Simple physics, related to opposing forces.
Amps with very poor damping factor (non OTL tube amps, for example) can't fight much against the back EMF of the speaker. This causes a "sloppy sound", especially noticeable in the bass. Also, most amps with great damping factor only have that characteristic at low freq (100Hz, where it's typically tested, and lower), then the output impedance climbs like crazy as the freq goes up. Some amps with high bandwidth damping attain this by applying tons of feedback, creating the need for wild phase shifts to allow stability. This is especially the case with Class-D that wraps feedback around the output filter. The same technique to get decent specs on paper when the amp is driven with sine waves of one or only a few frequencies (what's used for THD+N testing, for example). Music is not a continuous tone! The problem is that this same "high feedback" practice can cause an "etched" sound. How many times have you heard "great bass, but harsh sounding" about one of those pre-fab module based Class-D amps? We have conquered this issue with our patented and proprietary amplifier circuitry. This is why users of other Class-D amps, as well as tube fans, have migrated to Cherry Amps. Thanks.
-Tommy
