The reason I asked is, if the imbalance occurs & is mostly noticeble at lower listening levels then 9 out of 10 times it is the volume pot which is not tracking both channels equally. I've owned preamps that did this & the fix was to replace the volume pot. This is not an expensive fix & only another suggestion to look into.
The other possibility is that the tubes are not matched within acceptable tolerences producing unequal gain.
The other possibility is that the tubes are not matched within acceptable tolerences producing unequal gain.