Most guitar amps utilize push-pull output circuits. In a push-pull circuit one tube (or set of tubes) amplifies the positive half of the signal and the other tube (or set of tubes) amplifies the negative half of the signal. In many amps the same bias voltage is applied to the cathodes of all the output tubes. It is important that all the output tubes dissipate approximately the same plate current when a common bias voltage is applied to each of them. This is why your output tubes need to be matched for similar plate current dissipation, preferably over a range of approximately 30 to 70 milliamps.