• 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.

Best 12AX7

Yes, good enough technically speaking, my own data points prove it. Will it sound better with different tubes and OP? Good question...
With the MC275, they used the 12AZ7 possibly to accommodate the 6550 (Rg max = 50K). Interestingly, most/all the re-issues of this century use 12AT7.