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!
Hum and Hiss from Sonic Frontier Preamp
Last night was the first time I used my head phone jack from my SF line 2 pre amp. There's a subtle hum and an occasional hiss when the pre amp is on. Does any one have this problem or know what causes this problem? The only upgrade I made were changing the standard 6922 tubes to valvos tubes. Thanks in advance for all your help. Btw, there's a slight hiss when I play my front speakers as well, but it's not as noticable or as anoying as when I use the head phone.