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Try switching tubes and listen again. Both hum and hiss could be from there. Also check for a gound loop (hum). It seems clear to me, that you hear more of a hiss through earphones, there is no attenuation through room effects etc. Possibly, I am not familiar with this preamp, it is inherently a bit noisy, (tube hiss), but the on and off hiss you describe could be a tube, misbehaving.
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.
Some light tube "rush" is perfectly normal and is generally not noticable under typical listening conditions. Stronger hiss that comes and goes or varies in intensity or can be heard over program content could be caused by a number of tube related problems. If it's just one channel, try swapping each of the tubes left to right in sequence to isolate the offending tube. Excessive hiss from both channels would be a bit unusual - perhaps poorly graded replacement tubes generally (??? - Valvos are pretty good, though). Hum is an altogether different beast and is generally a grounding issue. Not that it's any excuse whatsoever, but quality headphones are much more revealing of these abnormalities than most speakers. Where hiss is involved, don't overlook your program material - many older masters have some intrinsic hiss. Also, try switching sources to make sure your woes are not originating elsewhere. Good luck!
I agree completely with Sgmlaw. I have a line three and there is a slight hiss that is audible if you place your ears next to one of the speakers with no active source. Increasing the volume control does not change the hiss amplitude appreciably which indicates that the hiss is part of the noise floor. I used to own an ARC LS22 and it acted the same way. I have no experience with solid state preamps but my experience with SS amps indicates that their noise floor should be inaudible. I am curious as to how the sound quality of a good quality SS preamp compares to my Line 3 (which sounds great).
It is my understanding that the SF Preamps use a solid state op-amp type headphone amplifier made by Headroom