This is an interesting review -
www.stereophile.com
and we can see that the amp did not meet the datasheet specs. First, I would like to comment on CCIF IMD 19+20kHz result:
@John Atkinson measured this:
and it should be
50W peak into 8ohms. Frankly, I do not like "peak power" definitions, so I have measured my 1ET400A at 40W/8ohm (rms) with this result:
It may be seen that the distortion components are at or below -105dBr. What a difference from Stereophile result near -70dBr, it is a huge 35dB!!
I am absolutely sure it is not a difference between 1ET400A and1ET9040BA, but the difference would be in case material. If the Buckey case is made from iron steel, then the coils in the Purifi module become nonlinear (nonlinear core) and the best way to test this is high level 19+20kHz IMD (not the usual 5W). Even 20kHz THD would not indicate the issue.

So, my cordial suggestion to @Buckeye Amps would be to try again with aluminium case. Maybe a bit more expensive, but worth results. Remember the dissimilar faston/binding posts materials.
Buckeye Purifi Eigentakt 1ET9040BA monoblock power amplifier Measurements
Sidebar 4: Measurements
and we can see that the amp did not meet the datasheet specs. First, I would like to comment on CCIF IMD 19+20kHz result:
@John Atkinson measured this:
and it should be
Fig.11 Buckeye Eigentakt 1ET9040BA, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 50W peak into 8 ohms (linear frequency scale).
50W peak into 8ohms. Frankly, I do not like "peak power" definitions, so I have measured my 1ET400A at 40W/8ohm (rms) with this result:
It may be seen that the distortion components are at or below -105dBr. What a difference from Stereophile result near -70dBr, it is a huge 35dB!!
I am absolutely sure it is not a difference between 1ET400A and1ET9040BA, but the difference would be in case material. If the Buckey case is made from iron steel, then the coils in the Purifi module become nonlinear (nonlinear core) and the best way to test this is high level 19+20kHz IMD (not the usual 5W). Even 20kHz THD would not indicate the issue.


So, my cordial suggestion to @Buckeye Amps would be to try again with aluminium case. Maybe a bit more expensive, but worth results. Remember the dissimilar faston/binding posts materials.