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The biasing of any tube stage depends on having the signal coupled into the grid of the tube at the proper DC level. A leaky coupling capacitor lets current through from the preceeding stage and upsets the DC bias, usually turning the tube on so hard that no signal can pass through it. A power tube with a leaky or shorted coupling capacitor may blow fuses cause low power or excess hum, or kill transformers. A preamp tube will just cause little or no signal to pass through or ugly-sounding distortion of the signal that does get through.
In all cases, you must determine whether the coupling capacitor is leaky. A quick way to test the capacitor is to unplug the output tubes, connect the (-) lead of your multimeter to chassis ground, and use the (+) lead of your multimeter to measure the voltage on the socket contact that corresponds to the grid. You must know whether your amp is cathode biased, in which case the grid contact must be at zero volts, not positive at all.
If your amp is a fixed bias amp, the grid contact(s) in the socket(s) must be at a negative voltage, -15 to -60Vdc. If they are more positive than -15, you probably have a leaky coupling cap.
If either of these tests indicate a leaky or shorted coupling cap, open the chassis, determine the value of the suspected coupling capacitor, and replace it with another of the same or higher voltage and capacitance rating. Then button the amp back up and see if the voltage on the socket contact for the grid has changed.
It's best to make this substitution with a capacitor you don't mind leaving in if that was NOT the problem so you don't have to go back and pull it back out.
It is possible to unsolder the grid end of the coupling capacitor, turn the amp on and measure the DC voltage from the unconnected end of the capacitor to ground with an analog voltmeter. If you only have a Digital Multi Meter (DMM), connect a 1M ohm resistor from the free end of the capacitor to signal ground, then measure the voltage across the resistor. If this voltage is even 1V, the capacitor should be replaced; the capacitor is leaky. This is a definitive test, but you do have to have the amp open and powered on, and so it is a more hazardous test.
For all tubes except cathode follower stages and output tubes in guitar amps, this means that the grid of the tube is tied to ground through a high (over 470K, often 1M) resistor. Cathode follower stages have their grids tied either directly to the plate of the tube driving them or to a fixed positive voltage.
The tests for a leaky coupling cap in a preamp tube are the same as for power tubes, except that you have to know which tubes may be direct coupled cathode followers. A direct coupled cathode follower stage will have it's grid tied directly to the plate of the driver tube ahead of it, no coupling capacitor at all. This is only the case for some older Fender Bassman models and a number of Marshalls. If you find a tube socket with a high (could be 100V or more!) positive voltage on it's grid contact, unplug and open the amp, and drain the B+ capacitors. Then look at the grid circuit in question. If there is a capacitor between it and it's driving circuit, the coupling capacitor is bad. If it is connected by either a wire or a resistor to the driving circuit, it is direct coupled and the high grid voltage is not a defect.
For all stages that are not cathode followers, the voltage on the socket pin for the grid should be dead zero, not positive even a fraction of a volt.
Replace coupling capacitors with at least 600v rated capacitors regardless of their original ratings unless you simply can't get such caps.
If neither of these fixes the problem or materially changes the symptoms, back up a step and make another guess.