I don't know about him, the lowest I have ever seen are those measured on ASR, the top two are the AHB2 and LA90.
Worse case only:
Benchmark AHB2 (about 0.003%/-90 dB 90 kHz bandwidth) and
Topping's LA90 (about 0.0007%/-103 dB 45 kHz bandwidth). Both are class AB amps.
Those %/-dB levels were for 90/45 kHz bandwidth, for 20 kHz, the numbers will obviously be a bit lower, especially if measured with the commonly used 1 kHz sine wave signal:
AHB2: 0.0003% at rated 190 W into 4 ohms, both channels driven
LA90: 0.00018% at 56 W into 4 ohms, both channel driven
Expect much better/lower numbers in the common 1 kHz rated output into 8 ohm test.
For a rough comparison, the best Yamaha measured there was the WXA50, better than the RX-A3080 and the CX-A5200, as well as the A-S801 measured by Gene:
Worst case:
about 0.17%, 22 kHz bandwidth
The popular
1 kHz test at rated output only:
55 W at 0.06%
50W at 0.003%
We both know such numbers have little practical meaning for most people in most cases, but they do tell us something about the "engineering excellence" and build quality of the products.
To some extent (and only to some extent), it is like you know a fully equipped Toyota Camry is practically as good as its comparable Lexus, and in a blind test (auto pilot mode engaged obviously) you may not be able to tell them apart, but you will still happily pay more for the Lexus version.