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.