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.
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.