The specs stipulate: "less than 0.05% between 20 Hz and 5 kHz and 0.5% between frequencies 5 kHz - 20 kHz. We have to figure that they meant less than 0.5% as well for this latter frequency range. Sorry for my misunderstanding.
No problem, it is always confusing when the specs are stated without the details. By nature, amplifier distortions would vary with a few things, including the test frequency and the output level, among others.
In most cases, when compare specs you can only compare distortions at rated output with test signal at 1 kHz. In this case, Monolith provides much more, as much as or more than ASR's.
Their less than 0.5% 5-10 kHz is actually not bad, when you compare with other amps.
But first, take a look of the 1 kHz test that is typically included in amp specs:
Sequence Report (monoprice.com)
So you can see that the worse channel was channel 8, but still at only 0.002095%, 20-22,000 Hz. That's a lot lower than 0.5%. They may prefer to just say less than 0.5% in case some samples might have higher distortions due to manufacturing tolerances of certain parts/components, and/or they allowed for some load conditions that could be worse than those used during the bench tests. It could also be that they want to be on the conservative side. You can contact them for their rationale, I really shouldn't be guessing.
Now take a look of the THD+N vs test signal frequency:
Now you can see that the worst performing channel had THD+N of a little over 0.1% at 20 kHz, that's much higher than when tested with a 1 kHz signal but still below 0.5%.
For reference, take a look of the measurements of an AVR such as the Marantz SR7015:
The graph is arranged differently but you can also see much higher THD+N at higher test frequencies.
Note that the curve for test frequency higher than 10 kHz is not that important. When tested at 15 kHz, the harmonics will be in the ultrasonic band that you cannot hear anyway.
In this example, the SR7015 specs say 0.05% at rated output 20-20,000 Hz, but the measurements showed much lower levels at 1 kHz, about -89 dB, or 0.0035%.
