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.
The newer 1700V devices look like they have better SOA at the ROI, and they cost a lot less (5-10 dollars vs. 80 dollars). Check out the data sheets at Digi-Key. I did a search of 1700V devices up to ~7A rating.