These low distortion levels (at full load) would look pretty good on any hifi audio amplifier. But this particular amplifier had to handle more than a 1000 times faster signals. I developed this amplifier in 2004 as part of a large industrial T&M instrument. The full power signal bandwidth was 30MHz (not 20kHz) and if I remember correctly the signal swing was something like 40Vpp in 20 ohm. As any experienced designer will attest, the technology to achieve this level of performance at these frequencies is not supposed to be available, not in the 21th century anyway.