I don't know all of the usual diagnostic steps you may want to take. I do know as a tubophile, that tubes can be the source of a hiss. If you can swap out the tube/s in the preamp section you may find your answer very quickly.
You should also be aware that tube equipment is inherently noisier, not that all tube systems are noisy. What I am trying to say is that if you are very sensitive to background noises some tube systems will never be quiet enough for you.
You should also be aware that tube equipment is inherently noisier, not that all tube systems are noisy. What I am trying to say is that if you are very sensitive to background noises some tube systems will never be quiet enough for you.