!CAUT!ON! Resistors normally get very hot. Use caution and always let a unit cool before attempting to check the resistors.
Carbon-composition resistors should be measured out of tone control and phono circuits and replaced with low-noise, 1 % tolerance carbon or metal-film types. Sometime metal-film times can add an "edge" or "harshness" to the sound, so it's best to experiment.
Resistors are are relatively inexpensive. A lot cheaper than replacing vacuum tubes and not a lot more effort or work, once the soldering iron is heated up.
Carbon-comp resistors change value as they age. You can check this by using an ohmmeter and comparing the reading to the stated value of the resistor. If the resistance reading is off more then 5% of it's specified value, it should be replaced. Don't forget to reference against the schematic, as the resistor may have been previously replaced (it is also possible for the schematic to be incorrect).
Carbon-comp resistors can also develop thermal irregularities. They work and measure fine on the bench, but once warm or hot they may change value and develop instability, resulting in objectionable noise. Resistors sometimes work fine till hot, then go completely open.
Some circuits require a high degree of resistor balancing or matching in order to function properly. Such example circuits are:
Fundamentally, if these resistors are not closely matched, no amount of pot-twiddling or tube matching is going to help the amp sound better.
Match the resistor values by buying a batch (ten or twelve) of extra resistors for a specific value. Measure the entire batch and pick the most closely matched values. It sounds like extra work, but you ears will appreciate the effort.