What speakers are you using, what's their sensitivity? It's not uncommon to hear some audible hiss, particularly if your speakers are sensitive - say 94dB or higher, perhaps.
You might be able to find quieter input tubes.
I have the Titan A100 and it's dead quiet into my 90dB ProAcs, but I'm sure if I hooked it up to my 96dB Voxativ I would hear some audible hiss.
If it's slight, and barely audible from the seat, it's probably considered normal...