Any tube will red plate if it’s drawing too much current compared to the other tube in pair if you can’t bring it’s bias within the same range for the amp’s bias voltage requirements.
In set or cathode bias amp the cathode is connected to ground via a resistor which is bypassed with a capacitor and applies a positive voltage to the cathode, the grid is connected to ground with a resistor and causes the grid voltage to be negative with respect to the cathode. This turns out to be a very reliable method of maintaining bias and extending tube life but may have audible disadvantages and also requires that all the tubes in a set are very closely matched.
The problem with good quality "NOS" tubes such as the EL34 is they’re becoming scarce and the ability to find closely matched quads can be difficult and expensive.
In fixed bias a constant negative voltage from it’s own power supply is supplied to the control grid; by varying the grid voltage, you can change the current flow between the cathode and the plate fine tuning the pair’s output characteristics ensuring they operate efficiently. This would allow more variability when choosing a quad since they could be less closely matched.
Excessively large differences in a tube’s voltage characteristics though, might exceed the range of the amp’s bias adjustment causing one or more tubes to draw more current than the others resulting in excessive current draw from the amp’s power supply.
You should consult with your dealer as to the type of bias your amplifier uses and choose new tubes accordingly.
