"Many circuits for studio gear specify 1% components."

For resistor values, yes, and capacitors in frequency sensitive bits like EQ circuits, but not absolute maximum voltage ratings only 1% above normal working voltage, which is what we are talking about here.

Valves are pretty tolerant of overvoltage and excess power dissipation, more so than transistors. I doubt that valve heaters are all that bothered about a few percent over voltage either.