Most probably (99%) a disbalance resides in slightly different (or even not so small) REAL electric specs of the tubes. It happens even for the NOB's, not even speaking about a weared second-hand. That is why many people pay extra for selected pairs, since a special testing device measures a set of electric specs for each tube and we can make the most equal pairs.
If disbalance is not so big and your audio source has a precise L/R balance control (ALL modern digital sound cards and operating systems offer this feature), make a realtime recording, measuring the signal strength for L and R, adjusting it through OS mixer or special sound driver feature.
Even more, some people don not recommend to use full sound card amplification (so called 100%), using 70-80% interval, where you can adjust your disbalance easily. Do not forget, that our amd is really powerful and you can compensate a volume drop turning an amp's knop instead of amplifying a sound card noises.
Tell us, if it helped.