WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Well, if it's just coupling caps between stages or on inputs, I don't see why you can't vary the value. It really wont make a difference to the amplifier except drop the frequency response by a hair or much less... I'd try it for sure!