Resistor Drift and Noise

While a lot of people like carbon composition resistors in their vintage amp, there is one issue you need to watch out for.
This is especially true in amps where the valves hang upside down as in the  original black-face and silver-face Fender amps.
In these amps the heat goes upwards and straight into the components.  The output valves are the hottest lace in the amp. The result is that the resistors get cooked over time.  When this happens the resistor values start moving higher. I have seen a 470 ohm 5 watt resistor end up at 660 ohms after 40 years. It is no shame on the resistor manufacturer, however, when restoring an old amp it is best to check the resistor values in all the hot places in any amp and replace any values that are out of specification or are near the high end of the tolerance.
De solder one end and check the vlaue with a multimeter and replace as ncessary.

Cabon resistors tend to generate more  internal noise than modern metal film resistors.  In older  high gain amps, it may be worthwhile to change any carbon resistors around V1, especially the anode resistors.