There are several reasons as to why a bias circuit resistor could fail. Most likely, I'm guessing at this point without a schematic in front of me, it sounds as if one of your output tubes is pulling grid current. If the output tubes are old, this becomes even more likely. Depending on the resistor type, total failure or significant value drift can occur simply from thermal cycling (heating and cooling). Remember, most of the low wattage resistors used in audio cost less than 5 cents (in bulk, much less). I'd replace the resistor with similar wattage and type, and monitor the voltage drop across it with the same output tube in place. As long as the voltage drop is the same as the other bias/grid resistors, then chalk it up to a faulty resistor.
VTL used to sell a "white book" that had the basic circuit schematic that they used in almost of their amps. They were very straight forward as I seem to recall and should be quite easy to troubleshoot.
Good luck.
