Hi Marek,
The symptoms you've described appear to point to the line stage as being the culprit, and I suspect in particular the first tube in the signal path within the line stage.
If you can determine that separate input tubes are used for the two channels in the line stage try interchanging those tubes and seeing if the problem moves to the other channel.
Before doing that, though, it would probably make sense to interchange the cable connections between the two channels at the outputs of the line stage, to verify that the problem is caused by something "upstream" of that point. If the problem moves to the right speaker when you do that, restore those connections to their original state and then interchange the cable connections between channels at the inputs of the line stage. If the problem then does NOT move to the right speaker you will have pretty much verified that the line stage is the culprit.
Have everything turned off while you are changing any connections, of course.
All the best to you and yours as well, for the holidays and the new year!
Best,
-- Al
The symptoms you've described appear to point to the line stage as being the culprit, and I suspect in particular the first tube in the signal path within the line stage.
If you can determine that separate input tubes are used for the two channels in the line stage try interchanging those tubes and seeing if the problem moves to the other channel.
Before doing that, though, it would probably make sense to interchange the cable connections between the two channels at the outputs of the line stage, to verify that the problem is caused by something "upstream" of that point. If the problem moves to the right speaker when you do that, restore those connections to their original state and then interchange the cable connections between channels at the inputs of the line stage. If the problem then does NOT move to the right speaker you will have pretty much verified that the line stage is the culprit.
Have everything turned off while you are changing any connections, of course.
All the best to you and yours as well, for the holidays and the new year!
Best,
-- Al