HI fujindemon
the tubes may have low hours on them ... but they were probably takenout -- shipped to you and put in --- tthey probably bounced around alot ....
and that is a good idea fromRmml --- try switching the tubes on the amp from left to right and also the preamp if the amp had no effect ... remember-- if you do not switch them back you need to rebias ....
probably a good idea after all is done just to rebias everything
stephen