WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Not much to add but I'ld be interested to hear about the results if you decide to test them further. I've got a few of those and their specs would make them suitable for a project. IIRC d3a was one of the best distortion-wise in your test.