I have heard that DSLs can have that problem but I don't know what the root cause is.
I think there is a suspect CAP on tube plate circuit...you may want to do some Google searches on it because I found several Forums discusing this issue in the past.
I have also heard that if the power tubes are not burned in correctly that they can drift.
FWIW...All of the above is hearsay as I have not experienced that problem myself.
