You were thinking the same thing I was . It's likely that a previous service attempt had been the lowering of that resistor's value as that would raise the HV . Thing is , the lack of brightness would be far more likely to be the 2.5 ohm resistor that is now up over 6 ! If the HV rectifier's filament doesn't receive the power it needs no amount of lowering the resistance on the high side is going to get any more HV out of the tube . I'll bet you see a great improvement when you get an actual 2.5 on the filament and go back to the 470 K on the HV filter .