Sounds like you have a tube going bad. For starters, swap the left channel tubes with the right channel tubes and see if the distortion moves to the other channel. That will confirm that the issue is in the tubes.
next, you can simply replace the bad channel tubes, or replace all of them if there are not too many and the tubes are reasonably priced.
next, you can simply replace the bad channel tubes, or replace all of them if there are not too many and the tubes are reasonably priced.