NC252MP is below 0.004 ohm, and 1ET400A, although it varies more than the NC252MP, is below 0.001 ohm, both through the audio range.
Putting them into context, for example:
Hegel H160: below 0.1 ohm.
"The output impedance was very low over most of the audioband, at 0.07 ohm, rising to 0.1 ohm at 20kHz. (Both figures include 6' of speaker cable.) As a result, the modulation of the H160's frequency response with our standard simulated loudspeaker (fig.1, gray trace) was minimal."
https://www.stereophile.com/content/hegel-music-systems-h160-integrated-amplifier-measurements
Krell KSA-50S: below 0.28 ohm.
"its output impedance was highish for a solid-state design at a calculated 0.28 ohms. This was the same at 20kHz as it was at 20Hz, however. "
https://www.stereophile.com/content/krell-ksa-50s-power-amplifier-measurements#
CH Precision M1.1: below 0.1 ohm best case.
"The output impedance with 100% feedback, including a 6'-long, spaced-pair speaker cable, was a low 0.1 ohms across the audioband. With 20% feedback, it rose to 0.24 ohm at low and middle frequencies, and 0.27 ohm at the top of the audioband."
https://www.stereophile.com/content/ch-precision-m11-power-amplifier-measurements#
McIntosh MC275: below 0.38 ohm best case.
"The McIntosh's output impedances were on the low side for a transformer-coupled tube design, at 0.33 ohm from the 4 ohm tap, 0.57 ohm from the 8 ohm tap, and 0.87 ohm from the 16 ohm tap. These figures applied at low and midrange frequencies; the impedance at 20kHz was a little higher, at 0.38 ohm (4 ohm tap), 0.7 ohm (8 ohm tap), and 1.1 ohms (16 ohm tap)."
https://www.stereophile.com/content/mcintosh-mc275-power-amplifier-measurements