oh man, that is hilarious! at his expense unfortunately, i am going through the exact same thing!
one thing he did not do was change the bias voltage or lower the B+.
i think i might add some cathode resistors of maybe 50 ohms or so on each pair to guard against runaway. once the left or right pair of power tubes starts to conduct more than the other, it gets warmer, which lowers the plate resistance, which means more current, more heat, which also drags the pwr supply nodes down like he experienced, i do not think today's tubes can be run at old school specs, if they do not fail then their life will certainly be short compared to NOS RCA etc.
i usually check output transformer ratios with the variac on the primary side as to no generate dangerous voltages, and i load the secondary with a resistor. if that guy would have done that, his voltage on the secondary would have dropped, thus yielding a different impedance ratio that might have been easier to shop for. he might have got 5K:8 or 5.2K:8 instead of 5.8K:8.
thanks Greg!