The maximum value of Grid #1 resistance applies to both original production as well as modern production tubes of any given type. Since modern tubes seem to be more trigger happy relative to their own self destruction, most feel that it applies even more to modern production tubes.
The issue involves an effect known as "reverse grid current" which can occur from a number of reasons -- with all of them basically a product of heat, or too much of it. If the heater is operated at too high of a temperature, or out-gassing occurs due to exceeding dissipation specifications, it can all lead to the #1 grid effectively becoming an emitter of current within the tube. When that happens, there is a slight current drop across the gird #1 resistance (more commonly known as the grid return resistor, or the 300K resistance in this case), that works to produce a positive voltage acting against any negative bias voltage applied to the resistor. This can produce a run away condition, since reducing the negative bias voltage causes the tube to draw more current producing more heat, causing more reverse grid current to flow, causing more tube current, and so on. To limit this run away condition from occurring, a maximum value of Grid #1 resistance is specified for most tubes, limiting the amount of reverse bias voltage that can be developed. Since cathode bias is a self regulating form of bias, the maximum value of grid return resistance is greater for cathode bias designs versus fixed bias designs.
It is important to understand that the maximum value of resistance specified is based on the tube operating at maximum rated dissipation levels; as a tube is operated at lower dissipation levels, then the value can be safely exceeded -- which is how Fisher (and many others) got away with exceeding the value. Theoretically, if the tube is operated at only 70-80% of its maximum dissipation ratings, then the maximum resistance can be exceeded somewhat. However, with today's higher line voltages and therefore higher heater voltages, it can cause cathode material to boil off and deposit itself on the grid. If that happens, hello negative grid current. Or if the tubes are not matched closely enough in the original circuit, allowing one tube to be a current hog and run very hot -- hello negative grid current. Or if the tubes are not properly ventilated, hello........ It all goes back to heat and what happens when there is too much of it.
Reducing the 330K grid return resistance to about 220K is a very good move in the stock circuit and operating conditions to add some protection for the tubes. Let's face it, the suckers pretty much cooked due to a variety of reasons. However, as IBAM type circuits are installed (equalizing the current for all tubes), the tubes are adjusted to operate comfortably below rated dissipation levels, care is taken to ensure that the heater voltage is within reasonable limits (5%), and truly proper ventilation is provided, then the need to reduce the value of these resistors nearly vanishes. When new back in the day, Fisher could control the match on the tubes initially installed in the set, and AC line voltages were an established, lower value to design for. The original manufacture tubes were also much cooler in terms of current draw relative to a given bias voltage, so exceeding dissipation ratings were generally not a problem. The main problem Fisher had to be concerned with then was temps when the unit was installed in a case. It is a testament to the quality of the original manufacture tubes that so many lasted as long as they did in that environment without catastrophic failure
Today however, with much hotter running modern manufacture tubes (relative to bias voltage), notably higher AC line voltages, and a potential mish-mash of tubes installed (i.e., unmatched), then operation with the stock design plugged straight into today's AC power with the unit installed in a case can spell certain doom. Lowering the value of these resistors is critical in this case, and likely still won't be enough to save the unit and its tubes from disaster. If all the common sense guidelines are followed however, lowering the value of these resistors won't hurt, as long as the coupling caps are adjusted accordingly as well.
I hope this helps!
Dave