We did indeed get a cranky batch of 431's some time ago. Any replacement that doesn't have "WS" etched onto the chip will be a suitable (quiet) replacement. Do note that changing the cathode resistor under the 12AT7 in the Stereomour will not change the plate current.
You would find that the 7062 will achieve lower plate voltage in a stock Stereomour. If we assume PJ's optimal plate voltage value is 175V, you have a 500 Ohm cathode resistor, and your C4S is setup for stock, the cathode resistor/C4S combo will give you 1.8V of bias. 1.8V of bias and 3.6mA lands you somewhere around 120 plate volts on the characteristic curves.
3V of bias and 8mA (110 Ohm R1, 375 Ohm cathode resistor) of current looks like a pretty nice region of operation, and this gives you about 180 plate volts. The 300K/3W R2 resistor perhaps should be changed to something more like a 200K/3W. The issue with the above scenario would be the extra heat dissipated by the MJE5731A transistors, which could be dealt with by adding a heatsink to each of them.
The other option is to use more bias (4V) and less current than above (3.6mA, stock). This could be achieved with a 1K cathode resistor to tweak the bias, and you would be running under thermal operation conditions consistent with the design.
In an ideal world, you would try both and report back!
