My Line 3 exhibits a slight hiss, which is completely inaudible except when listening directly at a speaker with no music playing. I am relatively new to the tubed world, but I do not believe this issue to be uncommon, as I have heard it from other tubed pre-amps. All this talk may not make it right, however. Hopefully someone will chime in and provide an explanation for tube hiss, or say that I am experiencing a problem as well. Either way, we'll both learn.
The hum I would say is a different issue, and a fair guess would be it has nothing to do with the pre-amp or it would probably be audible through your speakers as well. Most sources of hum can be traced to ground/earth issues, and while I don't use them, it wouldn't surprise me that the problem exists in the headphone section of the pre-amp, or with the headphones themselves. Did you contact SF to see what they say? I would be interested in their response as well. While responses via email can be slow coming, they have always responded to my inquiries.
My opinion, for what it’ssss worth.
The hum I would say is a different issue, and a fair guess would be it has nothing to do with the pre-amp or it would probably be audible through your speakers as well. Most sources of hum can be traced to ground/earth issues, and while I don't use them, it wouldn't surprise me that the problem exists in the headphone section of the pre-amp, or with the headphones themselves. Did you contact SF to see what they say? I would be interested in their response as well. While responses via email can be slow coming, they have always responded to my inquiries.
My opinion, for what it’ssss worth.