You will get some tube hiss but should only be audible when you put your ear up to the speakers; say about a foot away. If it is audible from your listening chair, say 10+ feet away, then you probably have a noisey tube. Does it come from one channel specifically or both? I believe that unit only has one tube per channel, two total, so you may want to repace both especially if it is a used unit and you don't when the tubes were last replaced.
Chuck