See http://headroom.headphone.com/RootNavigation/home.asp
This is a feature of the SF preamps (see their web site) as Headroom is known for very high quality headphone amps. I don't think they use tubes for the headphone functions, but the tubes might buffer the signal prior to the hitting the headphone amp. I have a Line 2 and it does not have the headphone problems you are experencing. My SF used with the BAT VK-200 amp has very little "hiss" when the volume is all the way down, by the way. I would say that the headphone output is very quiet, quieter than the line output. I would also offer this: I had one bad tube in the Line 2 which caused excessive hissing out of the right speaker but had absolutley no effect on the right channel of the headphone (or left channel for that matter). This would appear to indicate that at least some tubes are not part of the headphone circuit, and that tubes may not be the root of the problem. I am guessing but you may have a problem with the headroom amp part of the preamp. If you are using low-impedance headphones, perhaps the current drive from the circuit is not sufficient? Have you tried different headphones? Anyway, that's my input I hope it helps you out.